Theo proved the matchwinner on Sunday.
Pioli said: “Theo had put himself at our disposal as a central defender and he did it very well indeed, but naturally he has such a great rapport with Leao down the left, they know each other so well, can sniff each other out.
“I think Theo struggled last season to get over the disappointment of the World Cup Final defeat, the strain of a tournament that was in the middle of the season. I was not sure how he would be mentally this season, but he was very positive. He has become a leader for this team, not one who talks that much, but with his attitude.
“I think the games he played as a centre-back also showed him how much more focused, aggressive and important he can be when defending, and he is taking that into his role as a left-back now."
He added: “I had many meetings with the players over the last few weeks, demanding they show more concentration and be more solid in defence, less ready to immediately press high. The strikers also need to send the signal and spark attacking moves to be accompanied.
“Tonight's Milan was balanced, organised and above all worked hard, as Napoli forced us to work hard, and we did not hold back. Keeping a clean sheet in those circumstances should give us more confidence going forward."