Despite taking a third-minute lead through Nani's clinical strike, the Reds rarely approached top gear as the tenacious hosts overturned their deficit and clung on for a deserved victory.
"We are disappointed, particularly after going in front so early in the game," Sir Alex told MUTV. "The result was caused by bad play at set-pieces. That is the nuts and bolts of it and we don’t dispute that.
"It was a great start. We made a lot of chances, particularly in the first-half. Nani had two great shots at the goalkeeper, Wayne Rooney had a good chance too, so we played a lot of good football in the first-half and made some great chances, but we were never at the races in the second-half. We had an incredible amount of possession. The pitch wasn't very good, but nevertheless we never really created anything."
The league leaders' cause was hindered by the late withdrawal of Rio Ferdinand, who suffered a calf injury during the warm-up and is now out of action for a fortnight. In his stead, Jonny Evans started, while Chris Smalling stepped up to the bench.
"Normally I think we would have put Chris Smalling in that position because he has been playing right side and Jonny has been playing left side, (but) Jonny had already done the warm-up. It was important because of the experience that Rio has in games like that."