Ole Gunnar Solskjaer has dismissed David de Gea's mistake against Watford as "one of those things" after his side suffered a 2-0 defeat on Sunday.
Watford scored twice in the early stages of the second half to seal the three points against the Red Devils.
De Gea was held responsible for Watford's first goal as Ismaila Sarr's shot bounced past him and into the top corner before Troy Deeney converted a penalty.
The loss is United's first in seven matches in all competitions and leaves them eighth in the Premier League, drifting further behind their rivals for a top-four finish.
Despite the costly setback, Solskjaer praised De Gea's effort for United and says he still has confidence in him.
"It's one of those things," he told BBC Sport. "He's been so good in training and focused.
"One goal is a mistake and the second is penalty. David's been very confident lately and I don't look at it any other way."
He continued to Sky Sports: "It was a mistake. It happens in football and it is what we train for every day to make sure they don't happen again.
"David has been very good lately, it is just one of those things that happen.
"You have to earn every point on the pitch and today we didn't deserve that. I can talk about it all day long. We have to face the fact we have two games coming up that we have to improve on."
United attacker Jesse Lingard missed United's biggest opportunity in the first half when he went through on goal but tried to chip it over Watford goalkeeper Ben Foster and missed the target.
Although the coach feels the attacker made the wrong decision, he held off criticising him.
"Ben Foster stood up so I don't think a chip was the right option," he added. "It was a good run, we could have been 1-0 up but it happens.
"Sometimes you miss the target and sometimes you score.
"We need a reaction."
United are in action again on Boxing Day, when they host Newcastle at Old Trafford before they visit Burnley two days later in their last game of 2019.