Manchester United midfielder Nemanja Matic was forced off early in the second half with what appeared to be a leg injury.
Ander Herrera replaced him in the 54th minute of the contest at Vicarage Road with the Red Devils up 3-0.
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Matic has started all 14 of Manchester United's Premier League contests this season since his €45 million move from Chelsea during the summer transfer window, providing one assist for Jose Mourinho's side.
The severity of the injury is unknown, but there appeared to be little reason to take any chances with the midfielder as a first-half double by Ashley Young put United up early against Watford, with Anthony Martial adding a third before the half.
An injury that sidelined Matic for any period would come at an inopportune time. United have an important run of fixtures in the next two weeks with a Premier League showdown at Arsenal coming Saturday. That is followed by the final Champions League group stage matchday against CSKA Moscow midweek and then a derby date with league leaders Manchester City the following Sunday.
A victory over Watford would shrink the deficit to their Manchester rivals to five points with the Citizens having an opportunity to re-extend it to eight points against Southampton on Wednesday.