Ryan Giggs says United only have themselves to blame following their Champions League defeat to Celtic.
The result means the Reds need at least a point from their final Group F match at home to Benfica on 6 December to ensure qualification for the knockout stages.
Despite dominating proceedings at Celtic Park, the Reds failed to take their chances and were made to pay by a superb free-kick from Shunsuke Nakamura nine minutes from time.
"We only have ourselves to blame," Giggs told MUTV. "We had a lot of the ball and we had the chances. Celtic hardly got our of their own half in the first half.
"But when you are on top, you have to take your opportunities and we did not do that."
United were handed a lifeline in the dying seconds of the match when referee Mejuto Gonzalez pointed to the spot following a hand ball by substitute Shaun Maloney.
However, Louis Saha's subsequent penalty was brilliantly saved by Celtic's Polish keeper Artur Boruc.
"It was unfortunate for Louis but we should have had the game wrapped up long before then anyway," insisted the Welsh winger. "Now we have to play Benfica again.
"It won't be easy, we are aware of that. But maybe one thing in our favour is that we know Benfica have to win, so they will have to come out and attack us.
"But we are at home and we know what we have to do. Hopefully we can," he added.