Spanish sports paper Diario AS reports this Friday 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 they 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 Thursday with the aim of being in condition to play the return leg, but there is no way he will be able to play in the Vicente Calderon Saturday night.
Ronaldo's absence will be a huge blow for Real Madrid, for although the Champions League is more important that the derby, the truth is that Jose Mourinho's side wants 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 of the 24 his side have netted against their 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 meanwhile must look on the game as their best chance over the past 14 years to beat their neighbours and climb back into second place in the BBVA Primera Liga.
Striker Diego Costa is available to play after seeing the yellow card in his side's 1-0 win against Sevilla at the weekend lifted and is certain to be a handful for the Real Madrid defence.
Costa is hard-working and strong, but also an expert in some of football's 'dark arts' and has the ability to provoke even the most even tempered of rivals. He will partner Radamel Falcao in the Atletico attack and few will have forgotten Real Madrid' s reported interest in the Colombian striker earlier in the season.
German newspaper Bild published Thursday that both Falcao and Mourinho would be joining Chelsea over the summer and the fact that the Real Madrid coach and the Atletico Madrid striker (who share the same agent, Jorge Mendes) could find themselves at the same club next season, adds another ingredient to a match which so often promises to much but has delivered little.
Will this time be any different?