Moments later it was almost 3-0, Gosling arriving late into the penalty area but curling his effort just wide from a Sarr cutback.
Rovers pushed forward as the half wore on, Foster tipping a low shot wide and then Kabasele clearing a dangerous cross, but the pressure was not sustained.
Just before the interval Gosling got his goal, finishing low after Cleverley fed Sarr down the right flank and the winger cut another pass back.
Watford made ten changes at the interval to give minutes to as many outfield players as possible, with only Foster retaining his place between the sticks.
Cucho Hernández began the second half positively, driving directly towards goal and feeding Kwadwo Baah for another opportunity, but the forward’s shot from an angle was blocked.
Emmanuel Dennis was withdrawn as a precaution following an injury shortly after, and the winger was replaced by a trialist on the left flank, with Baah moving to the right.