Leao scored twice in a 4-0 away win over the runaway league leaders, with Brahim Diaz and Alexis Saelemaekers also getting on the score sheet.
Leao had gone eleven games without a goal for Milan but showed the kind of form that saw him win the Serie A player of the season last term.
The 23-year-old has now scored 10 goals and provided six assists in 26 Serie A appearances this term.
Asked whether Leao was 'back' after his double, Pioli told DAZN: "He never left!
"He scored two goals, one from a central forward position and one from out wide. He has incredible potential and will become a champion."
It will not be long before the two sides meet again; this time in the Champions League quarterfinals next week but Pioli is not expecting this result to have a significant impact on their subsequent meetings.
"Certainly today's victory gives us confidence, but they will be different matches," Pioli added.
"Just because we won 4-0 today, it doesn't mean that Napoli have lost their certainties. We'll think about Napoli and face those two matches – I hope – with the personality of this evening.
"It will be a balanced match between two strong teams who are doing well in the Champions League. This season will be positive if we play in the Champions League again next year.
"There are still 10 days to go. It will be 180 minutes, but now we head to Empoli [on Friday] – in the league we haven't always faced matches with the right attitude."
