The attacking duo struck either side of half time to earn a win in something of a smash-and-grab raid against a Hornets side who were the better team for much of the game.
Abdoulaye Doucouré pulled one back in the 89th minute, capping a flowing team move to claim a deserved consolation effort for the visitors.
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Having missed England’s Euro 2020 qualifiers, Rashford was fit enough to be included form the start, while Martial made his first league start since early February.
Watford Goalie, was a spectator for much of the opening half hour, as the visitors caused United all sorts of problems, with front two of Troy Deeney and Gerard Deulofeu looking lively.
It was typical then, that Foster’s first significant involvement was to pick the ball out of his own net, as Luke Shaw launched a brilliant counter attack with a stunning curled ball in behind for Rashford, who finished calmly to take his league goal tally into double figures for the season for the first time in his career.
This is the first time since the 1995-96 season (Scholes, Cole, Giggs, Cantona) that Man Utd have had four players score 10+ goals in a single Premier League campaign (Martial, Rashford, Lukaku, Pogba).
The goal sparked some life into what had been a lethargic United display, and Martial would have made it two from close range, were it not for an instinctive stop from the legs of Foster.
There was a nervy ending at Old Trafford when Doucoure pulled one back late on but United held on to move back into fourth place ahead of Arsenal – who are in action against Newcastle on Monday.