* Alvaro Arbeloa kicked Casillas while clearing a ball
*Casillas was controversially dropped by Mourinho
Iker Casillas is a major doubt for Real Madrid's Champions League double-header with Manchester United after breaking a bone in his hand.
The World Cup winner suffered the injury in his side's Copa del Rey quarterfinal victory over Valencia on Wednesday when teammate Alvaro Arbeloa kicked him while clearing a ball in the area.
Madrid will assess the injury before deciding what treatment is required while Adan Garrido is likely to deputise for the Spanish No 1.
Casillas will miss Sunday's La Liga clash with Getafe and the second leg against Valencia next week.
He is also also struggle to recover in time to face United at the Bernabeu on February 13.
'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 said.
Meanwhile, the club said in a statement: 'After carrying out initial x-rays... it has been confirmed that the footballer Iker Casillas has suffered a fracture at the base of the first metacarpal on his left hand.
'It will be assessed again by a specialist later today to decide what treatment will be followed.'
The injury to a key figure comes at a delicate time for Mourinho with Europe's elite club competition resuming in just over two weeks.
Casillas has just been restored to the starting line up after the manager 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 defence has started to tighten up of late.
The 2011 Copa del Rey winners will face a high-profile clash with great rivals Barcelona if the holders can beat Malaga in their quarter-final second leg at the Rosaleda.