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 today's draw for the Champions League semifinals.

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 domestic rival Barcelona out of the competition on Wednesday.

However, Atletico president Enrique Cerezo confirmed there is a clause in the loan contract that means the team 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 Barcelona, 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 how much was involved, Spanish media reported it would be around 3 million euros (US$4.15 million) 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 was so pleased with the keeper that it wanted him to stay at the Vicente Calderon past this season.

If he is prevented from playing, Atletico would have to turn to reserve Dani Aranzubia, 34, who has played just once this season.

Atletico remains the only unbeaten team among the final quartet and is back in the semis of Europe's elite competition for the first time since 1974, when it reached the European Cup final but lost to Bayern Munich after a replay in Brussels.

On Wednesday, Adrian Lopez filled in for injured striker Diego Costa and created the fifth-minute goal after hitting the post, as the Atletico forward headed an ensuing cross on to Koke to fire home in Madrid.

Barcelona dug in but produced few tangible scoring chances apart from a pair of first-half misses by Lionel Messi.

The teams had played four previous games this season which all ended in draws, but Atletico's breakthrough kept Barcelona from the semifinals for the first time in seven seasons.

"We just have to keep working hard and stay calm because we still haven't won anything," said Atletico coach Diego Simeone, whose team will face either crosstown rival Real Madrid, Chelsea, or Bayern in a two-legged semifinal. Bayern beat Manchester United 3-1 at home on Wednesday to eliminate the English champion 4-2 overall.