Cristiano Ronaldo may miss Saturday's derby as Real Madrid travel across town to play Atletico and could also be a doubtful starter for his side's Champions League semi-final return leg on Tuesday.
Spanish sports paper Diario AS reports that Ronaldo suffered a muscle problem in his left leg as he warmed up for Wednesday's first leg in the Champions League, a game in which Real were comprehensively outplayed by Borussia Dortmund, who won 4-1.
Ronaldo was below his best in that game, but although he scored, he hardly celebrated and ended the game with obvious signs of pain.
He began treatment on the injury on Thursday with the aim of being in condition to play the return leg, but is not likely to feature at the Vicente Calderon on Saturday night.
Ronaldo's absence will be a huge blow for Real Madrid as Jose Mourinho's side looks to maintain their record of not losing to their neighbours since 1999 and thus assuring they remain in second place in the BBVA Primera Liga.
There is also the fact that should FC Barcelona win away to Athletic Club Bilbao in a game which finishes just before the Madrid derby kicks off and Real Madrid lose in the Calderon, Barca will be proclaimed as League champions for this season, inheriting Real Madrid's crown from last year.
Atletico have been one of Ronaldo's favourite 'victims' since arriving in Spain and has scored eight goals against the local rivals, who are desperate to finally have the chance to end their 14 year jinx.
Mourinho will make other changes for the derby and players such as Kaka, Raul Albiol, Jose Callejon and perhaps B-teamers, Nacho Gonzalez and Casimiro, could get their chance as the aim is to try and overturn that three-goal deficit against Borussia.
Atletico have their best chance in over 14 years to beat their neighbours and climb back into second place in the La Liga table.