Norwich took the lead in the second half through Anthony Pilkington's superb header and then hung on to claim a famous three points, against a relatively poor United side.
The loss knocks the Red Devils off of top spot in the Premier League and, while being disappointed with the result, Sir Alex chose to praise the hosts.
He told ESPN: "We're disappointed to lose the game obviously but we have to give credit to Norwich, the way they fought, battled for every ball on the pitch, and they defended fantastically well. The goalkeeper had a really good night also, so it's disappointing."
The United boss also hailed the quality of the Canaries' goal, which came about after a fine move and a inviting cross from Javier Garrido.
|16/1||Manchester United are 16/1 with PaddyPower to finish outside the Premier League's top four|
He continued: "[It was a] marvelous cross, marvelous touch for the header and nothing the goalkeeper could do about that. But as I said I think they deserved the win just simply because they worked so hard for it."
United came from two goals down to win at Villa Park last weekend and Sir Alex felt tonight could have been the same.
The Scot said: "Although we had a lot of possession and one or two half-chances without having great chances and it just wasn't our night.
"I always feel we can do that, the history of the club's like that and the players we've got are used to making comebacks, particularly in the last minutes of matches, we're always a threat."
The Red Devils were also without Wayne Rooney for the match but Sir Alex believes there was enough quality present for his side to do well.
He added: "You always miss your good players, there's no question about that, but we had plenty of good players on the pitch tonight."