The Rossoneri are two goals down in the tie ahead of Tuesday’s game having been comfortably beaten by their rivals last week, in a defeat which Leao missed due to a thigh injury.
Pioli kept the Portuguese forward out of his squad for the weekend defeat at Spezia but will now recall him after a successful return to training.
“Rafa is fine,” he confirmed to reporters.
“He did nearly all the training session yesterday, today we hope he completes it.”
Milan are also hopeful of welcoming back Rade Krunic and Junior Messias from injury issues, Pioli added.
Given their deficit, the coach is hoping to create a siege mentality for his squad as they bid to pull off one of the all-time great Champions League comebacks.
“It's not easy to prepare for tomorrow,” he continued. “But we have an important goal: to win the match and to qualify for the final, which nobody would ever predict.
“Nobody still thinks that Milan can reach the final. I know we can play a great match.”