United won the game 1-0 at the Etihad, but lost the semifinal 3-2 on aggregate.
"I like a lot the way they played," said Guardiola.
"At Old Trafford we played really well, maybe I didn't expect us to play the way we played, but I said when we lost here I said they don't need much.
"The quality of the players up front is incredible. I had the feeling, I said before when we lost at home against them, they are trying to do what the manager Ole wants, and I think it works.
"Maybe they are not consistent in all the games, they struggle at home a little bit with the teams who defend deep.
"The people up front they are so fast. Matic and Fred, they are incredible players, Matic is an outstanding player. In time, they will do it and be back to what United was for many years."