Cristiano Ronaldo left Real Madrid's pre-Champions League training session early on Tuesday after colliding with Kiko Casilla
Madrid were training ahead of Saturday's San Siro showdown against city rivals Atletico - a repeat of the 2014 final which they won 4-1 extra-time.
The Portuguese collided with the goalkeeper before getting to his feet and walking from the field, shortly before the session was due to end.
The issue did not appear serious, but will be an unwelcome development for Zinedine Zidane as the club go in search of an 11th European triumph.
In a media conference earlier on Tuesday, Zidane confirmed Ronaldo had sat out a practice match on Saturday as a precaution after "a little bit of a feeling in his leg".
The 31-year-old has been in superb form in this season's competition, scoring 16 goals in 11 matches as he aims to break his own record of 17 set in the club's victorious 2013-14 campaign.