The Inter Milan manager's comments came following United's 2-0 Champions League tie success against his side at Old Trafford on Wednesday night.
"Right now they're at their maximum level," he said. "They have experience, quality, great physical condition and intensity in their game.
"That's why they're European champions. That is why they can win everything. I think they can win five trophies.
"I'm honest in my way of analysing things and they are better than us.
"My team played to their maximum of their potential. They couldn't have given any more."