The Canaries took the spoils through Anthony Pilkington's neatly-converted header, a goal from which United rarely looked like recovering, even in a season already laced with rousing comebacks.
"We're disappointed to lose the game, obviously, but we have to give credit to Norwich for the way they fought and battled for every ball on the pitch," the boss told ESPN.
"They defended fantastically well and their goalkeeper had a really good night also. It's disappointing. They deserved the win just because they worked so hard for it."
United had previously rescued 15 points from losing positions in the Barclays Premier League this term, and Sir Alex admitted that he always felt his side were in with a shout of rescuing another lost cause.
"We had a lot of possession and one or two half-chances without having any great chances, but it just wasn't our night," he said. "The players we've got are used to making comebacks, particularly in the last minutes of matches.
"We're always a threat. We were in some respects tonight also, but they defended really well and the goalkeeper's made two or three really good saves at vital moments."