January signing Bruno Fernandes opened his account for Manchester United as the Reds climbed into fifth in the Premier League table.
Fernandes had been felled in the penalty area in the first-half and dispatched his spot-kick well. French striker Anthony Martial then produced some individual brilliance to double United's lead after half-time. A third came from 18-year-old Mason Greenwood who now has 11 goals for the season.
Chelsea's Saturday victory against Tottenham Hotspur opened up a gap between fourth-place and Manchester United. But the win for Solskjaer's side means they now sit only three points off Chelsea and one point ahead of both sixth-placed Tottenham and seventh-placed Sheffield United.
Story of the game
It wasn't until a Mason Greenwood long-range effort that either goalkeeper was challenged but chances were aplenty at both ends. Harry Maguire and Nemanja Matic conspired to leave Troy Deeney racing through on goal in the third minute. Only a well-timed intervention from an ever-improving Luke Shaw could deny the English forward.
Doucoure struck into the sidenetting after another loose ball from United offered Watford an opportunity. It was the theme of the first half. But so too was the quality of Fernandes, soon to grab his first goal for the club.
The Portuguese midfielder has settled in perfectly at Old Trafford. His arrival off the bench in Bruges, Belgium in midweek hastened the pace of United's game. This time he started and showed his quality. There were a couple of excellent floated balls to full-backs Shaw and Wan-Bissaka, the occasional driving run through the middle and the constant movement between the lines and off the shoulder of the last defence.
It was Wan-Bissaka who had the best chance of the early stages, striking wide after Ben Foster palmed out a Dan James cross. Shaw was denied from long-range and soon after Martial missed his first chance, launching an effort on the outside-of-the-box over the bar. He'd been fed through by Fred but the move had started when Shaw, under pressure in his own box, passed out well. The left-back has looked better with each game in the last couple of months.
Watford's intent to expose United didn't stop and it was Doucoure who surprised De Gea with an early left-footed drive towards the inside of the left post. The Spaniard matched the powerful effort and palmed it out for a corner.
Though Fred impressed as a midfield force again, he missed a golden opportunity after a cut-back from Dan James. Fernandes had opened space in the area, James found Fred and the Brazilian curled an effort over the left side of the goal.
The disappointment felt around Old Trafford was shortlived. Four minutes later and four minutes before half-time, Fernandes tapped it past Foster who blatantly brought him down. The January signing demanded the spot-kick and deserved it after a good first-half. With a hop just as he pulled back to strike, he deceived Foster and put it into the bottom right corner.
United had been fine in the first-half but lost possession too much. Watford would capitalise on that briefly after the break. Deeney netted at a goulmouth scramble from a corner but the VAR intervened to rule that it had come off Craig Dawson's arm and ruled out the equaliser.
It was a lucky escape for United but individual brilliance from Martial quickly made the most of it. After being put through by Fernandes, the Frenchman's first effort was blocked by Foster. Martial regained control and made to cut back towards a teammate before a quick Cruyff turn gave him space to shoot and chip Foster in front of the Stretford End.
United cruised from there. Greenwood was denied by Foster after five stepovers took him past his marker. Maguire headed wide from a perfect Fernandes cross. Then Greenwood provided his own quality, driving forward on the halfway line, playing a one-two with Fernandes and rifling home a typically lethal, powerful strike.
With quarter of an hour remaining, Odion Ighalo came on for his Old Trafford debut, against his former side. Tahith Chong and Scott McTominay both came on too with the later returning from a two-month injury.
Ighalo struck the post within a few minutes, intercepting a poor header back to Foster and going round the English 'keeper. Masina and Foster recovered and Ighalo could only earn the corner, off the post.
Chong was slipped through by an excellent Matic pass but a curled effort squeezed outside the left post.
Takeaways from the game
Fernandes gives United everything they've lacked
It was a match memorable for Bruno Fernandes' first goal in a United shirt. It came from the penalty spot but the Portuguese was consistently excellent. He moves the ball quickly, stays aggressive in the tackle and drives forward well. And to add to that, his penalty and the way it was converted showed that he's a bit of a showman, something the Old Trafford fans always like.
United's full-backs improve
Shaw has put together an impressive run of performances. He's adding a lot more in attack but it's in defence where he's impressing, covering well for the centre-backs and doing a good job in the air. Wan-Bissaka, meanwhile, has added more in attack. He looks more confident going forward and that's a big change.
Stand-out player: Bruno Fernandes
The 26-year-old was at the centre of everything for United. His attacking contribution is unmissable but he manages to keep the tempo up for United. In defence, he's an aggressive force even if Fred and Matic were left to do most of the work in that department.