Real Madrid goalkeeper Iker Casillas' suspected broken thumb during the Copa del Rey second leg against Valencia could rule him out of both legs of the Champions League last-16 tie against Manchester United.

Casillas was forced off after 17 minutes during the 1-1 draw on Thursday and will undergo further tests to reveal the extent of the injury.

The Madrid captain, a veteran of over 500 appearances for Jose Mourinho's side, has been dropped from the first-team on two occasions this season, for the defeat to Malaga and the win over Real Sociedad.

And the Spaniard could be forced into spending as long as two months on the side-lines at a crucial stage of the season, with the Copa del Rey semi-final and Champions League last-16 tie against United on the horizon, the only two remaining trophies Madrid can realistically contest with the club 15 points being Barcelona in La Liga.

Iker Casillas
Casillas could be out of the United Champions league ties.

"I immediately knew something was wrong," Casillas said. "I'm in a bit of pain. The doctor doesn't want to make any hasty judgements.

"He's seen something he doesn't like and we'll have to wait to analyse it in greater detail. We are going to perform the relevant tests to see the seriousness of the injury. I hope my injury is not serious.

"Because it's a finger we have to analyse it properly. Let's hope the results are positive but it's the doctor who gives the orders. When I tried to close my fist I felt that it hurt.

"It's just the thumb and the fact that it has nothing to do with the wrist is good. I will try to be fit again as soon as possible."

The news comes a boost to United's hopes of overcoming Madrid over two legs on 13 February and 6 March, as both Sir Alex Ferguson and Mourinho chase their third European Cup triumph.

The match-up represents the stand-out tie of the first knock-out phase of the competition between two of the most decorated sides in European football history, with Madrid and United having won 51 domestic titles and 11 Champions League's between them, while the club's domination of the Deloitte's Football Money League, where the clubs are placed 1st and 3rd respectively, signifies the enormity of both sides on the world stage.