Leao, 23, has been one of Milan's most influential players this season with 13 goals and 10 assists in all competitions, but he lasted only 12 minutes of Saturday's 2-0 win over Lazio before being taken off.
Pioli said Leao would either start the game or not play at all, adding that Belgian winger Alexis Saelemaekers was a potential replacement for Wednesday's match at the San Siro.
"Today he (Leao) trained, tomorrow I will decide what to do," Pioli told reporters, adding that he was not stressing over his fitness.
"I'm going to bed quietly tonight, anyway in the morning Rafa and the doctor will tell me his condition. If he is well he will be called up, otherwise not.
"If the test is clean he can play. If it is not clean he cannot play - either from the start or the end. I would have preferred to do it today, but it was not possible."
Milan, who were dethroned as Serie A champions last week by Napoli, have had a rough run in the league with only three wins in their last 10 games to slip to fifth - two points behind fourth-placed Inter.
However, Pioli chose to focus on their European run during which they knocked out Tottenham Hotspur in the last 16 and Napoli in the quarter-finals.
"We are talking about Champions League, we have not had ups and downs but an outstanding run," he said.
"We want to try to overcome this last step before winning a Champions League final, something that hasn't happened to all of us. Motivation is at its highest.
"It is the most difficult, because it is a Champions semi-final. But it is also beautiful to prepare, to experience, to play. I have tried to transmit concentration and happiness to the team to live these moments."
