UEFA.com rounds up the comings and goings at all 16 UEFA Champions League clubs, offering an expert opinion on how their winter dealings might shake things up.
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In: Gabriel Paulista (Villarreal CF), Krystian Bielik (Legia Warszawa)
Out: Benik Afobe (Wolverhampton Wanderers FC), Lukas Podolski (FC Internazionale Milano, loan), Yaya Sanogo (Crystal Palace FC, loan), Joel Campbell (Villarreal, loan)
What they needed: Greater solidity down the spine of the team – Gabriel should provide that – and for their treatment room to start to clear.
Reporter's view: Gabriel's arrival is just what Arsenal fans have been crying out for. Almost as significant is the form of 23-year-old Francis Coquelin, who has been commanding in the holding role since returning on loan from Charlton Athletic FC. Bielik, 17, is one for the future.
Fantasy tip:Mesut Özil and Theo Walcott are back among the goals following long-term absences and Santi Cazorla is playing a more instrumental role than ever.
Club Atlético de Madrid
In: Fernando Torres (AC Milan, loan), Cani (Villarreal, loan)
Out: Alessio Cerci (Milan, loan)
What they needed: Some help for Mario Mandžukić, the Rojiblancos' main centre-forward who carries much of the side's goalscoring burden on his shoulders.
Reporter's view: Torres helps bring pace and proven quality to help ease the burden on Mandžukić, who has had to play a lot of games this season. Cani comes in as a useful squad player who can fill a variety of positions.
Fantasy tip: Diego Simeone favours a 4-4-1-1 formation with Antoine Griezmann, Atleti's top marksman in domestic competitions this term, given licence to roam in the No10 role just off the striker.
Barcelona are unable to sign players in 2015 due to a transfer ban imposed by FIFA.
FC Basel 1893
In: Adama Traoré (Vitória SC)
Out: Darko Jevtić (KKS Lech Poznań), Giovanni Sio (SC Bastia, loan), Naser Aliji (FC Vaduz, loan), Geoffroy Serey Die (VfB Stuttgart), Marcelo Díaz (Hamburger SV)
What they needed: Basel played with an attacking right wing-back in the first half of the campaign and have brought in Traoré to do likewise on the left. Alternatively, he can also be fielded higher up on the left. "He adds more flexibility to our game," said coach Paulo Sousa.
Reporter's view: Other than intelligent loan deals to prepare youngsters such as Aliji for the first team, and the offloading of the under-used Sio, perhaps Basel's most astute move of the window was to persuade defender Taulant Xhaka to sign a three-year contract extension, particularly with Serey Die moving to Stuttgart — his combativeness will be missed.
Fantasy tip: Breel Embolo, 17, is the young striking sensation everyone in Switzerland is excited about. He has already proven his knack for goals, is getting more and more playing time, and at 5m you might be in for a bargain.
FC Bayern München
Out: Xherdan Shaqiri (Internazionale, loan), Pierre Højbjerg (FC Augsburg, loan)
What they needed: Nothing really, as the lack of transfer activity showed. Bayern offloaded a fringe player and loaned out a young talent, while having no interest in bringing anybody in. "All our summer transfers have worked out," chairman Karl-Heinz Rummenigge recently said.
Reporter's view: Josep Guardiola's squad is already overflowing with talent and any more top-level signings would merely have congested an already competitive dressing room.
Fantasy tip:Guardiola is a big fan of mixing things up, so it is always difficult to predict his selections (just ask opposing coaches!). However, Juan Bernat seems to be a new favourite at the German giants, while Bastian Schweinsteiger appears set to return to more regular action.
In: Juan Cuadrado (ACF Fiorentina)
Out: Andre Schürrle (VfL Wolfsburg), Mark Schwarzer (Leicester City FC), Ryan Bertrand (Southampton FC), Mohamed Salah (ACF Fiorentina, loan), Tomáš Kalas (Middlesbrough FC, loan), John Swift (Swindon Town FC, loan), Lewis Baker (Sheffield Wednesday FC, loan), Marko Marin (RSC Anderlecht, loan)
What they needed: Very little – José Mourinho did the bulk of his shopping in the summer and showed such shrewdness that his team arguably look more settled than ever.
Reporter's view: Cuadrado will add more flair and dynamism and comes with the added value of also being able to operate at right-back if need be. The Colombian's arrival will place immediate pressure on Willian, while the departures of Schürrle and Salah underline Mourinho's determination to move on players quickly once he decides they do not figure in his long-term plans.
Fantasy tip:Diego Costa is yet to get off the mark in the UEFA Champions League this term, but his powerful displays domestically suggest it is only a matter of time.
In: Kevin Kampl (FC Salzburg)
Out: Ji Dong-Won (Augsburg)
What they needed: Dortmund don't so much require new players (except a world-class forward like Robert Lewandowski – but they don't come along very often), as for their current players to return to full working order. Kampl, presumably, has been picked out to one day replace Marco Reus, who is expected to move on in June.
Reporter's view: With Shinji Kagawa yet to rediscover his mojo and Henrikh Mkhitaryan also struggling for form and fitness, Kampl will force everyone to up their game in midfield. The Slovenian international could be the perfect foil for Reus.
Fantasy tip: Go for Kampl. He is new, he is ready to make his mark on the biggest competition, he is industrious, quick and gifted. Given that Salzburg play a system similar to Dortmund, he will not take long to acclimatise.
In: Paolo De Ceglie (Parma FC, back from loan), Stefano Sturaro (Genoa CFC, back from loan), Alessandro Matri (Genoa)
Out: Sebastian Giovinco (Toronto FC)
What they needed: Strikers Álvaro Morata and Fernando Llorente have both worked well for the team as attacking partners for Carlos Tévez, yet neither of them has scored on a regular basis this term.
Reporter's view: Recognising it would be difficult to improve the starting XI without huge investment, Massimiliano Allegri has opted to add more depth to his squad. Allegri knows Matri better than anybody, while Stefano Sturaro – a midfield warrior whose loan to Genoa has been cut short – will offer alternative options to an otherwise attack-minded midfield.
Fantasy tip: None of the three recruits is likely to be a frequent starter. Paul Pogba is shining and scoring on regular basis since Allegri switched to a 4-4-2 system with a diamond midfield.
Bayer 04 Leverkusen
In: Tin Jedvaj (AS Roma, loan made permanent)
Out: Levin Öztunali (SV Werder Bremen, loan), Philipp Wollscheid (Stoke City FC)
What they needed: When Jedvaj arrived in the summer, there were already those regretting the failure to make the deal permanent. After some very promising performances in the first half of the season, Leverkusen acted quickly to tie him down.
Reporter's view: Jedvaj's performances have been nothing short of remarkable given the Croatia defender's tender years, and the club will be delighted to have secured his services for the long term.
Fantasy tip: Emir Spahić and Ömer Toprak are both solid defenders and, at 4.5m each, could be superb value.
Manchester City FC
In: Wilfried Bony (Swansea City AFC)
Out: Matija Nastasić (FC Schalke 04), Scott Sinclair (Aston Villa FC, loan)
What they needed: Cover up front. Sergio Agüero's propensity for picking up niggling injuries meant Manuel Pellegrini went searching for another mobile alternative with an eye for goal. He got Bony, the leading scorer in the Premier League in 2014, no less.
Reporter's view: City have made one signing but it could prove significant with Ivory Coast striker Bony due in at the conclusion of the Africa Cup of Nations. Bony's pedigree is impressive. Some players who shine in the Netherlands – he hit 31 goals in 30 Eredivisie games at Vitesse in 2012/13 – fail to handle the step-up to the Premier League but not Bony. He struck 25 goals in 54 Premier League matches for Swansea and brings City formidable power, aerial ability and a predator's touch. His nine top-flight strikes this term exceeds Edin Džeko and Stevan Jovetić's combined efforts.
Fantasy tip:Frank Lampard could be tempting given his big-game experience, though Agüero remains one of the most guaranteed goal-getters in the competition and must be worth considering if fit.
AS Monaco FC
In: Bernardo Silva (SL Benfica, loan made permanent), Alain Traoré (FC Lorient, loan), Matheus Carvalho (CA Barra da Tijuca, loan)
Out: Borja López (RC Deportivo La Coruña, loan), Lucas Ocampos (Olympique de Marseille, loan)
What they needed: With Radamel Falcao, James Rodríguez and Emmanuel Rivière gone last summer, Monaco were bound to be in the hunt for a top international striker to complement Dimitar Berbatov.
Reporter's view: Silva's sparkling form makes the estimated €15m Monaco paid to render permanent his loan from Benfica look like a gamble that might pay off. The risks appear somewhat higher for the loan signings of the talented but injury-embattled Traoré from Lorient and unheralded Brazilian forward Matheus.
Fantasy tip: On a run of some 800 minutes without conceding in Ligue 1, Danijel Subašić is one of the in-form goalkeepers of the moment. He might have some saves up his sleeve in the last 16.
Out: Clément Chantôme (FC Girondins de Bordeaux)
What they needed: No specific requirements. "We will look around and we'll act fast if there is an opportunity to strengthen the team," said coach Laurent Blanc.
Reporter's view: Unable to buy without selling first, Paris's hands were largely tied in January, but a lack of urgency in terms of moving out players suggests Blanc is largely content with his resources.
Fantasy tip:The ever-improving Marquinhos currently gets the nod at right-back and will compete with Grégory van der Wiel. He might be a good bet.
In: Hernâni (Vitória SC)
Out: Kelvin (SE Palmeiras, loan), Daniel Opare (Beşiktaş JK, loan), Rolando (RSC Anderlecht, loan), Ivo Rodrigues (Vitória SC, loan), Otávio (Vitória SC, loan)
What they needed: Another right-back to act as Danilo's back-up, since Opare has left on loan and Ricardo Pereira is a winger.
Reporter's view: Despite rumours linking several stars with moves, Porto carry a settled squad into the last 16 and the biggest boost they will have in February will be the return of Yacine Brahimi from Africa Cup of Nations duty with Algeria.
Fantasy tip: Juan Quintero and Ricardo Quaresma were handed more playing time in the last month, Jackson Martínez remains prolific as ever, and midfielder Óliver Torres has recently shown an eye for goal.
Real Madrid CF
In: Martin Ødegaard (Strømsgodset IF), Lucas Silva (Cruzeiro EC)
What they needed: A defensive midfielder to compensate for an injury to Luka Modrić and fitness concerns over Sami Khedira.
Reporter's view: Both of the Merengues' additions are for the future, aged 16 and 21 respectively. Silva will add strength in depth to the centre of the park, while Ødegaard is quite the coup for the UEFA Champions League holders, given the Norway attacker was one of the window's most sought-after players.
Fantasy tip:Despite a sublimely talented midfield, Madrid rely overwhelmingly on their four forwards – James Rodríguez, Gareth Bale, Karim Benzema and Cristiano Ronaldo. Indeed the quartet are responsible for 87% of their goals in Europe and 82% in the league.
FC Schalke 04
In: Matija Nastasić (Manchester City, loan)
Out: Donis Avdijaj (SK Sturm Graz, loan), Felipe Santana (Olympiacos FC, loan), Christian Clemens (1. FSV Mainz 05, loan)
What they needed: Roberto Di Matteo's injury-prone defence has been below-par in recent months and were in real need of a centre-back: enter Nastasić.
Reporter's view: Nastasić is intended to stabilise things at the back. A one-time regular with City, the Serbian international has all the credentials to succeed in the Bundesliga.
Fantasy tip: Eric Maxim Choupo-Moting, a free summer signing, has profited from the switch to a 3-5-2 system. He has repaid his coach with goals and assists aplenty in the Bundesliga. At 6.5m, he could be a steal.
FC Shakhtar Donetsk*
What they needed: Shakhtar seem to have all positions covered and required no reinforcements.
Reporter's view: Top European clubs are rumoured to be tracking Douglas Costa, Alex Teixeira and Luiz Adriano, but Shakhtar look poised to keep their key men as they chase a sixth straight Ukrainian title.
Fantasy tip:Shakhtar's 4-5-1 template is well established, with UEFA Champions League leading scorer Adriano up front. The lone striker remains their biggest fantasy football star, though winger Teixeira is another fine Brazilian with bags of pace and ability.
*The Ukrainian transfer window does not close until 2 March.