Real Madrid forward Cristiano Ronaldo picked up an injury in Wednesday's 4-1 Champions League defeat at Borussia Dortmund and is out of Saturday's La Liga match at city rivals Atletico.
The Portuguese international has a "muscle problem" but the club are hopeful he will recover in time for Tuesday's semi-final second leg at home to Dortmund, assistant coach Aitor Karanka told a news conference on Friday.
"Before the Dortmund game he had physical problems and he won't be in the squad (on Saturday)," Karanka said.
"He did not train today," added the former Real player as he again stood in for Jose Mourinho who rarely gives the customary news conference the day before a league game.
"He has a muscle problem and he cannot help us but we are hopeful he can play on Tuesday," said Karanka.
Local media reported that Ronaldo felt a twinge in his left thigh during the warm-up before the Dortmund game although he played the whole match and scored Real's goal.
He is the Champions League's top scorer this season with 12 goals.
Ronaldo's absence against Atletico increases the chances of La Liga leaders Barcelona being able to wrap up a fourth title in five seasons on Saturday.
If Barca win at Athletic Bilbao in the earlier kickoff and second-placed Real lose at Atletico the Catalans will have an unassailable 16-point lead with five games left.