David Beckham limped off with a knee injury during the second half of Real Madrid's 1-1 draw against Getafe.
Beckham hurt his right knee after miscueing a cross and running into advertising hoardings behind the goal.
Jose Antonio Reyes, on loan from Arsenal, also injured his knee, his left, and both will miss Wednesday's Champions League game at Bayern Munich.
"They will not face Bayern," said coach Fabio Capello. "Reyes's injury is bad; Beckham's could be just as serious."
Reyes collided with Getafe goalkeeper Roberto Abbondanzieri and was stretchered off. Beckham will undergo a scan on his knee on Monday.
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And on a bad night for the Spanish giants, striker Gonzalo Higuain was also substituted with an injury late in the game.
Bayern, who trail Real 3-2 from the first leg, defeated Hertha Berlin 3-2 on Saturday.
Beckham had hit the crossbar with a 25-yard thunderbolt in the first half before he was replaced by Antonio Cassano.
Real lost ground in the Primera Liga title race, with new leaders Sevilla beating second-placed Barcelona 2-1 on Saturday. Real stay fourth with 44 points, but slip two points behind third-placed Valencia, who beat Celta Vigo 1-0 at home on Saturday.
Getafe dominated early on at the Bernabeu and took the lead with a well-taken goal from Daniel Guiza in the 38th minute.
But they only held that lead until just before the break when former Manchester United striker Ruud van Nistelrooy scored from the penalty spot.
It was one-way traffic in the second half but Real failed to press home their dominance. They have won only one of their last five home games.