LONDON (Reuters) - On-loan goalkeeper Thibaut Courtois is unlikely to play for Atletico Madrid against his parent club Chelsea if the two sides are paired together in Friday's draw for the Champions League semi-finals.
The 21-year-old Belgium international has been one of the stars of his side's rise to the top of La Liga this season and was outstanding as Atletico knocked Barcelona out of the competition on Wednesday.
However, Atletico president Enrique Cerezo confirmed there is a clause in the loan contract that means they would have to pay Chelsea "a lot of money" to field Courtois against the London side.
Speaking after Wednesday's 2-1 aggregate win over Barca, Cerezo said: "The problem is whether he can play or not if we come up against Chelsea. It could be that he cannot, the contract is the contract.
"If he is to play we will have to pay a large sum of money."
Although Cerezo did not specify exactly how much was involved, Spanish media reported it would be around three million euros (2.47 million pounds) for each leg.
Courtois joined Chelsea from Belgian side Genk in 2011 but was immediately loaned out to Atletico.
He has now played nearly 150 games for the Spanish side but none for Chelsea and Cerezo added that Atletico were so pleased with the keeper that they wanted him to stay at the Vicente Calderon past this season.
"We are trying to reach an agreement with Chelsea so he can stay with us for one more year," he said.
If he is prevented from playing, Atletico would have to turn to reserve Dani Aranzubia, 34, who has played just once this season.
Chelsea were expected to comment on the situation later on Thursday.
Atletico remain the only unbeaten team among the final quartet and are back in the semis of Europe's elite competition for the first time since 1974, when they reached the European Cup final but lost to Bayern Munich after a replay in Brussels.
The draw could throw up a number of intriguing meetings including a rematch of the 2012 Champions League final when Chelsea beat Bayern on penalties to lift the European Cup for the first time.
A Chelsea-Real Madrid match-up would see Blues boss Jose Mourinho trying to knock out the club he took to the semi-finals last season, while the Spanish team's manager Carlo Ancelotti would be hoping to overcome the side that sacked him in 2011.
The two clubs also met in the 1971 European Cup Winners Cup final, which Chelsea won after a replay in Athens.
Another intriguing match-up would be an all-Madrid semi-final derby which has also happened before at this stage of the competition in the days before the away goals rule, and needed three matches to complete.
They met in the 1958-59 season with Real winning the first leg 2-1 and Atletico he second leg 1-0 before Real won a playoff 2-1 in Zaragoza and then went on to win the fourth of their five successive European Cups when they beat Reims in the final.
The draw is being made in Nyon, Switzerland on Friday. The first legs of the semis are to be played on April 22-23 and the second legs on April 29-30 with the final in Lisbon on May 24.
(additional reporting by Iain Rogers in Madrid; Editing by John O'Brien)