"Following tests carried out on Kylian Mbappe today by the Real Madrid medical department, the player has been diagnosed with a thigh injury in his left leg. His progress will be monitored," Real said in a statement.
On Tuesday, Mbappe opened the scoring in the 10th minute in a 3-2 win in Bergamo to revive the holders' flagging Champions League campaign.
But the 25-year-old pulled up with a muscle pain after 36 minutes and was replaced by Rodrygo.
"Mbappe has some discomfort in his hamstrings. It doesn't seem serious but we'll see tomorrow," said Real coach Carlo Ancelotti after Tuesday's match.
"He couldn't sprint, it hurt him a bit and we thought it best to replace him."
Before pulling up Mbappe had one of his best outings in the Madrid jersey since joining last summer from Paris Saint-Germain with three clear scoring chances, including one in the second minute.
He scored the 50th goal of his career in the Champions League and his 12th since joining Real Madrid.
Real play Rayo Vallecano in La Liga on Saturday followed by the FIFA Intercontinental Cup final on December 18 and another Liga game at home against Sevilla four days later.
