Cristiano Ronaldo scored two in the first half to give United the win at home at Old Trafford.
“The first half performance you would have to say was superb, absolutely marvellous,” he told Sky Sports.
“But the second half was disappointing. I think they just took their foot off the pedal, relaxed and went down a gear.
“Saturday (against Everton) was a different issue all together. Tonight they were just far too relaxed. I think they enjoyed themselves too much.
“But we played very well (in the first half).”
He added on Dimitar Berbatov’s excellent piece of skill that put up Ronaldo for his second goal: “It was an absolutely magnificent piece of football. The balance and imagination – absolutely fantastic. It was a piece of football you’d pay double the money to see.
“Cristiano Ronaldo was in the right position to tap it in.”
