They can be the difference between glory and defeat, survival and relegation, between European football and missing out. We celebrate ten of the past two decades' shrewdest moves.
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Diego Costa (Atlético Madrid to Rayo Vallecano, 2012)
Recovering from a serious knee injury, the Brazilian-born forward needed games and Rayo needed saving. Ten goals in 16 outings almost single-handedly kept the Madrid outfit in the top flight, and Costa returned to Atlético a different player.
Nikola Žigić (Valencia to Racing Club, 2009)
Relegation loomed when the Santander side went knocking on Valencia's door for the services of out-of-favour Žigić, whom they had sold 18 months earlier. Playing alongside Pedro Munitis, the lanky forward picked up where he had left off at the club, his 13 goals in 19 games ensuring safety.
Jürgen Klinsmann (Sampdoria to Tottenham, 1998)
Another hero returning in his old team's hour of need. With Spurs battling relegation, the 33-year-old German scored nine in 15 appearances, including four against Wimbledon in a crucial 6-2 victory. The north London side survived and Klinsmann retired in style.
Márcio Nobre (Cruzeiro to Fenerbahçe, 2004)
Nobre marked his debut with a goal against Galatasaray, the first of 12 in 18 matches as the Yellow Canaries snatched the title. The deal was duly made permanent and, 13 years on, the forward holds Turkish citizenship and plays for third-tier Erzurumspor.
Robbie Keane (Tottenham to Celtic, 2010)
The ten-point deficit to Rangers upon Keane's arrival at Celtic was ultimately insurmountable but the Republic of Ireland's all-time top scorer had a good crack. A childhood Celtic fan, Keane netted 16 times in 19 outings and, despite only playing between January and May, was named Hoops' player of the year.
Edgar Davids (Juventus to Barcelona, 2004)
The Dutchman was a revelation in his short Camp Nou stay. When he came the Blaugrana were languishing in mid-table, but his superb displays propelled them to second place behind Valencia. Davids has been cited as the catalyst for Barcelona's dominance from that point on.
Andrea Pirlo (Inter to Brescia, 2001)
Pirlo was frozen out at Inter but, playing alongside Roberto Baggio and Josep Guardiola, he thrived at Brescia in a new deep-lying playmaker role. AC Milan spotted an opportunity and made him theirs. The rest is history.
Álvaro Recoba (Inter to Venezia, 1999)
Few gave Venezia much hope of extending their Serie A comeback beyond a season but they hadn't reckoned on Recoba. The Uruguayan was 22 and struggling to settle in Italy when he signed, but 11 goals in 19 games confirmed his talent. Venezia lived to fight another day.
Daniel Sturridge (Chelsea to Bolton, 2011)
Another starlet labouring to live up to the hype. Sturridge joined Bolton after failing to find the net in 13 substitute appearances for Chelsea that term; he proceeded to plunder eight in 12 outings as Bolton escaped the drop.
Christophe Dugarry (Bordeaux to Birmingham City, 2003)
A stint so influential that French World Cup winner Dugarry is now a Birmingham hall of famer – despite playing just 31 times for Blues. A relegation-defying effort of five goals in four matches towards the end of the campaign, all of them wins, did the job.