The pair tangled in the second half and television replays showed Carvajal clearly threw at least one punch at the Croatia international in an incident apparently missed by the referee.
Spanish media even suggested Carvajal had tried to bite Mandzukic's arm although it was impossible to tell conclusively from footage published on social media.
Real coach Carlo Ancelotti and Atletico counterpart Diego Simeone were asked about the incident at their post-match news conferences, and whether they thought European governing body UEFA might take action, but both said they had not seen it.
The return leg is at Real's Bernabeu stadium on April 22.
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Image: Atletico adrid's Mario Mandzukic walks off the pitch to receive treatment after sustaining a head injury.
Photograph: Juan Medina/Reuters