Real Madrid keeper and Captain Iker Casillas (Photo: Reuters)
Spain's number one was trying to collect a loose ball in the area during Wednesday's King's Cup quarter-final second leg in Valencia, when his team mate Alvaro Arbeloa accidentally kicked his left hand while trying to effect a clearance.
Casillas went off to be replaced by reserve keeper Antonio Adan early in the first half and the side went on to secure a 1-1 draw that put them into the semi-finals 3-1 on aggregate.
"When I tried to close my fist it hurt. It's just the thumb. The fact that it has nothing to do with the wrist is good," Casillas, who had his left hand heavily bandaged, was quoted as saying on the club website.
Real's assistant coach Aitor Karanka told the post-match news conference: "They have to do the pertinent tests, and when they have done those we will know what he has."
The injury to a key figure comes at a delicate time for Jose Mourinho's side with the King's Cup semi-final first legs due next week.
The 2011 Cup winners could have a high-profile clash with Barcelona, if the holders can beat Malaga in their quarter-final second leg at the Rosaleda on Thursday. The scores are level at 2-2.
In three weeks time, Real also have the visit of Manchester United in the Champions League last 16.
Casillas has just been restored to the starting line up after Mourinho controversially dropped him for the league games either side of La Liga's winter break.
The decision provoked angry reactions from home fans and fuelled media speculation about possible dressing room rifts even though Real's suspect defence has started to tighten up of late.
Slow to react
The champions have only conceded one goal in their last five outings in all competitions. In their previous five they had conceded 12.
Adan was at fault for the goal conceded in Valencia on Wednesday, a low free kick he was slow to react to, but Karanka defended him.
"Adan knows we have confidence in him," he said. "He played recently and was unlucky to get sent off. He knows he is young and has to progress, and that he has our backing."
The Cup and the Champions League have taken on extra significance for Real who trail La Liga leaders Barca by 15 points, and second-placed Atletico Madrid by seven as their title defence has faltered.
"This season, where we aren't going to win the league, we have to think about the Champions League and Cup," Mourinho said in an interview with a French radio station on Wednesday.
Barcelona will be looking to put last weekend's first league defeat of the campaign behind them when they host lowly Osasuna on Sunday (1800).
Atletico will hope to have leading scorer Radamel Falcao back from injury when they travel to face misfiring Athletic Bilbao in Sunday's late game (2000).
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