Cristiano Ronaldo is currently dealing with a thigh injury, but it is not expected to be serious.
UPDATE: Monday, Apr. 29, at 8:30 a.m. ET by Brandon Galvin
Exciting news as Cristiano Ronaldo (thigh) is expected to be ready for Champions League action, according to ESPN FC:
ESPN FC @ESPNFC
Real Madrid boss Jose Mourinho confirms Cristiano Ronaldo is fit for Tuesday's #UCL semi-final second leg against Borussia Dortmund. #RMCF2013-4-29 12:28:29
---End of update---
Fox Sports had the report (via Sky Sports):
Ronaldo, who has scored 31 goals in the Primera Division this season and hit a hat-trick in the corresponding fixture 12 months ago, sustained a minor injury to his left thigh during the warm-up before Madrid's 4-1 defeat to Borussia Dortmund in the first leg of their Champions League semi-final tie during the week.
He has undergone physiotherapy with the aim of being ready to face the Germans next Tuesday and will be rested against Los Rojiblancos.
While this sounds like a minor injury, the fact that Jose Mourinho and the team doctors are choosing to hold Ronaldo out against a noted rival like Atletico—who are only three points behind Real with five matches to play—should tell you how cautious Los Blancos are being with their star forward in an attempt to climb back from a three-goal hole against Dortmund.
Despite being without Ronaldo, Real assistant coach Aitor Karanka is expecting his side to come out motivated and ready to fend off Atletico Madrid on Sunday (h/t Real Madrid F.C. Twitter):
Real Madrid C. F. @realmadrid
#Karanka: “Excepto Cristiano Ronaldo, Marcelo y Arbeloa, el resto está bien y mañana saldrá el mejor equipo para ganar” #halamadrid2013-4-26 16:13:36
Ronaldo, Alvaro Arbeloa, Marcelo Vieira and Michael Essien (per the Fox report) are all expected to miss the match against Madrid on Sunday at the Vicente Calderon. Mourinho could also choose to rest other stars in anticipation of Tuesday's match with the German side.
In addition to his 31 goals in league play, Ronaldo has been deadly in the Champions League. He's one of the main reasons Los Blancos are in the semi-finals, and he scored Madrid's only goal in the 4-1 defeat to Dortmund in the first leg, bringing his goal total to 12 in 11 UCL matches.