Manchester United suffered a 2-0 defeat at bottom-placed Watford in the Premier League on Sunday as Paul Pogba made his first appearance for the visitors since Sept. 30.
United drew 1-1 with struggling Everton in their last league outing, while Watford were beaten 2-0 by league leaders Liverpool and entered Sunday's fixture with just one top-flight victory all season.
Though both sides struggled to produce moments of quality in the first half, it was United who went closest as Jesse Lingard was put through on goal and his chipped effort skimmed the roof of the net.
United's Daniel James was replaced by Mason Greenwood shortly before the hour mark, with Pogba coming off the bench on 64 minutes for Lingard. The France midfielder, who saw his return from injury delayed by illness in recent weeks, was unable to affect the result despite an encouraging display.
Of De Gea's error for Watford's opener, United manager Ole Gunnar Solskjaer said: "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.
"The big plus is Paul [Pogba] coming on, and that's the one plus. He added urgency, quality, some great passes.
"The boy has been working hard. He's fit and leaner. He'll bring us a lot. He's important ... he makes us create chances."
Solskjaer also rued his team's lack of urgency in the game where they dominated possession.
"The first half it could easily have been my testimonial. And it might have been a better atmosphere at my testimonial. It was subdued, slow, no tempo, urgency," Solskjaer added.
"We have started off something with the rebuild but today I'm disappointed because you have to come to these stadiums and get more points against teams like today."
Meanwhile, United captain Harry Maguire said: "We have to be better against these so-called lower opposition sides, who have a deep block.
"We were sloppy and gave the ball away numerous times. We were not good enough and we have to improve."
The match marked Nigel Pearson's second game in charge of Watford and his first win -- the Hornets' first home victory of the season.
The result leaves United eighth in the Premier league with 25 points from 18 games, seven points behind fourth-placed Chelsea.