Sources have told ESPN that the forward sustained the injury in a 50-50 challenge in training and stayed on the floor. He was examined by the club's doctors, who first feared a fracture. However, an MRI revealed no fracture, although the 23-year-old was still in pain.
In a statement released later on Monday, PSG confirmed Mbappe had received a knock, adding that another update on his condition would be made on Tuesday.
"Kylian Mbappé received a knock on his right foot in training today. He has undergone treatment this afternoon," read the release. "The medical tests are reassuring and another update will be made in 24 hours."
Mbappe is in a race against time to prove his fitness for the clash with Madrid at the Bernabeu on Wednesday, in which PSG hold a one-goal advantage after Mbappe scored a stoppage-time winner in the first leg last month.
He has begun treatment on his foot to ease the pain, but manager Mauricio Pochettino and his staff now fear that he will not be available for the match.
Any absence from Mbappe would be a major blow for PSG. Mbappe is the side's top scorer with 19 goals in 31 games in all competitions this season.