We look at the most eye-catching signings made by the group stage contenders, with the likes of Robin van Persie, Roberto Firmino and Son Heung-Min set to light up the UEFA Europa League.
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Allan (Udinese to Napoli): One of the biggest surprises of last season in Serie A, the 24-year-old Brazilian could become key to Napoli's new-look midfield. "He is very dynamic, he can tackle and has good technique; he is a complete player," said Napoli coach Maurizio Sarri.
Fernando Cavenaghi (River Plate to APOEL): The experienced Argentine striker arrived late in the month to help improve the Cypriot side's threat in front of goal and made an immediate impact, scoring twice on his debut on Sunday.
Fábio Coentrão (Real Madrid to Monaco): Coach Leonardo Jardim continues to add to his growing Portuguese contingent. Coentrão brings real quality and experience to Monaco's defence.
Abou Diaby (Arsenal to Marseille): Having been plagued by a series of injuries in recent years, Diaby is hopeful of reviving his career back in his homeland – might he even press for a place in Didier Deschamps' final UEFA EURO 2016 squad?
Roberto Firmino (Hoffenheim to Liverpool): The third most expensive signing in the Reds' history arrived after thrilling in the Bundesliga last term and has appeared in all four league games to date, starting the last two.
Raúl García (Atlético Madrid to Athletic Club): A Liga and UEFA Europa League winner and UEFA Champions League runner-up with the Rojiblancos, the 29-year-old brings a competitive edge and trophy-winning knowledge to the San Mamés.
Teófilo Gutiérrez (River Plate to Sporting): The Colombian international scored the winner on his debut – in the Portuguese Super Cup against Benfica – and struck twice in the UEFA Champions League play-off against CSKA Moskva. Could form a lethal strike partnership with Islam Slimani.
John Heitinga (Free agent to AFC Ajax): The 31-year-old former Atlético Madrid, Everton and Hertha Berlin defender has returned to his first club to provide much-needed experience to Frank de Boer's youthful squad.
Son Heung-Min (Leverkusen to Tottenham): A genuine goalscoring threat with plenty of European pedigree; should help to provide the support Harry Kane so clearly craves.
Pierre Højbjerg (Bayern München to Schalke, loan): A versatile box-to-box player with considerable international experience despite still being only 20, Højbjerg should really improve Schalke's midfield options.
Charles Kaboré (Kuban to Krasnodar): A Ligue 1 title winner with Marseille in 2009/10, the Burkina Faso midfielder has moved across the city after catching the eye over the past two seasons. He should hit the ground running.
Nikola Kalinić: (Dnipro to Fiorentina): The Viola continue to play an attacking brand of football under new coach Paulo Sousa, and Kalinić could be the goalscorer they so badly lacked under Vincenzo Montella. Went close to lifting the UEFA Europa League with Dnipro last season.
Ricardo Kishna (Ajax to Lazio): Already with two Dutch caps to his name, the 20-year-old's talent has never been questioned and opened his account on the opening day of the Serie A season against Bologna.
Delvin N'Dinga (Olympiacos to Lokomotiv Moskva, loan): The Auxerre academy graduate helped Monaco to promotion in 2012/13 before further proving his worth by winning two Greek titles with Olympiacos. Lokomotiv have the option to make the loan permanent.
Ricardo Quaresma (Porto to Beşiktaş): A veteran of some of the biggest clubs in Europe – including Barcelona, Chelsea and Internazionale Milano – the UEFA Champions League winning winger returns to the Turkish club for a second spell after his 2010 to 2012 stint.
Roberto Soldado (Tottenham Hotspur to Villarreal): Failed to transfer his prolific Valencia form to the Premier League but already seems reinvigorated with the Yellow Submarine, scoring in both the club's opening Liga games.
Ivan Tričkovski (Al-Nasr to Legia Warszawa): The FYR Macedonian international returns to Europe with something to prove and will be tasked by Legia to fill the hole left by Orlando Sá.
Robin van Persie (Manchester United to Fenerbahçe): The Netherlands' record goalscorer arrives in Istanbul to lead the line after 11 years in England which brought 144 Premier League goals.
Jelle Vossen (Burnley to Club Brugge): Brought in by Michel Preud'homme to replace the goals of the departing Tom De Sutter. The 26-year-old has plenty of scoring experience in Belgium after finding the target with regularity for Genk from 2006 to 2014.
Julian Weigl (1860 München to Borussia Dortmund): Weigl was expected to be a squad player but has had a phenomenal impact in his first few weeks at Dortmund. Only 19, he continues to be preferred to several more established stars.
Dimitar Berbatov (AS Monaco to PAOK): Arguably the biggest signing in the history of the club having helped Monaco into last season's UEFA Champions League quarter-finals, the 34-year-old Bulgarian will look to increase PAOK's attacking threat with his eye for goal, composure, precision and technical skills.