Real Madrid forward Gareth Bale could miss crucial games after suffering from a recurrent calf injury.
The club have described the problem as a "grade two" injury, but have not given a time frame for his absence.
Spanish newspaper AS has reported the Wales forward, 27, will be out for three to four weeks.
Real face Atletico Madrid in the first leg of their Champions League semi-final next Tuesday, with the final in Cardiff on 3 June.
Bale, 27, came off after 39 minutes in Sunday's El Clasico defeat by Barcelona.
Real, level on points with La Liga leaders Barca and with a game in hand, have important league games this week, facing Deportivo on Wednesday and Valencia on Saturday.
Wales play a World Cup qualifier in Serbia on 11 June, eight days after the Champions League final.
However, Bale is suspended for the match in Belgrade.
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