Portuguese winger Leao missed the 2-0 first-leg defeat and also sat out Saturday's shock loss to Spezia in Serie A after sustaining a thigh injury in Milan's league win over Lazio earlier this month.
Leao has been one of Milan's most influential players this season and registered 13 goals and 10 assists in all competitions.
"He is better, as are (Rade) Krunic and (Junior) Messias," Pioli said when asked about Leao before Tuesday's return leg.
"If everything goes as it should, they will be available for this afternoon's training and for the game tomorrow."
Pioli said he is confident Milan will be able to pull off a comeback at the San Siro after last week's dispiriting first-leg defeat in which they conceded twice inside the opening 10 minutes.
Seven-time European champions Milan have not reached the Champions League final since their last triumph in 2007.
"We have an important goal in mind - to win the game and to qualify for the final, which nobody would have predicted, just as at the beginning of the year no one would have predicted our presence in the semi-finals," Pioli said.
"We haven't talked about Spezia, only about tomorrow... We know we are starting with a disadvantage, but we also have the quality to overturn it.
"Now more than ever, nobody thinks Milan can reach the final. I know we can play a great game and I know how strong my players are... We've made some mistakes, but the past is irrelevant."
