The Rossoneri were beaten 3-2 at San Siro after having taken the lead a couple of minutes in through Ronaldinho.
"It's not finished. Tonight we proved that when we play our game, we caused Manchester United a lot of problems," insisted Leonardo.
"There was a portion of the match when we struggled and they scored two goals. The first half could easily have ended 3-0 for Milan.
"We were superior in large swathes of the game, but when we took a breather Manchester United made the most of it. When they played well, they took their chances, whereas we created so much and only scored twice."
The Coach has regrets at the way Milan let in two goals in quick succession from Wayne Rooney.
"It's a shame we didn't go into the break ahead, especially conceding in that way, as it affected us psychologically. A team cannot dominate for 90 minutes against a side like Manchester United, that is natural.
"The fact we were in control in the first half led to a slight wavering in the tempo we had set. If we had made more of our domination in the first half, it would've been a very different game.
"United were not playing better when they scored their second goal, it was an incident that changed everything.
"We had more chances at the end with Ronaldinho and Inzaghi, but did not reap what we sowed in terms of performance. We only scored two and deserved much more, while we did not deserve to let in three."