Louis Saha has apologised for his penalty miss in the 1-0 defeat to Celtic on Tuesday night.
The United forward could have secured qualification to the knockout stages had he converted the 89th-minute spot-kick, but his miss meant United lost and must now avoid defeat to Benfica in two weeks to go through.
"It is one of those things," he told Sky Sports. "I said sorry to the lads, but it is in the past and it will not change.
"We were surprised about the free kick and it is a shame as it would have been very nice to get a draw."
A point would have put United through, but for the second season running the Reds go into a final tie with Benfica hoping to secure qualification to the last 16.
Despite the disappointment of the defeat at Parkhead, United now turn their attentions to league meetings with Chelsea, Everton and Middlesbrough before the Portuguese side visit Old Trafford on 6 December.
Benfica can qualify only if they win, but Saha is confident the Reds can get the draw or victory they need to reach the knockout stages.
"Benfica have to come to Old Trafford so it is our advantage," he added. "We need a point and it is going to be a tough game, but we have a big advantage.
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