Real Madrid full back Alvaro Arbeloa has injured his right knee and is unavailable for Sunday's La Liga match at home to Levante.
"He has a knee problem and he has to rest for a few days," coach Carlo Ancelotti told a news conference on Saturday.
"After that we will see if he can train," added the Italian.
Local media reported Arbeloa would be sidelined for three weeks, meaning he will miss Real's Champions League last 16 return leg against Schalke 04 on March 18 and the "Clasico" against Barcelona five days later.
"It is not good for us because he is a very important player, not only for his defensive ability but for his experience and personality," Ancelotti said.
"We hope he recovers well and is fresher for the next matches."
Real are a point clear of champions Barcelona at the top of La Liga with 12 games left.
Barca have a chance to overtake their arch rivals if they can win at struggling Real Valladolid later on Saturday (1500 GMT), while Atletico Madrid are three points behind Real in third and play at Celta Vigo (1900).