Evans, who scored twice for the Wales Under-21 side as they disposed of Hungary's in a 4-1 victory while United slipped up, struck in the 62nd minute on his debut to crown an impressive display
Jamie Ward drew first blood in the 11th minute before Evans, forced out of Manchester City by Mark Hughes' multi-million pound signings, added a second against a Watford side who look to be contenders to spend the season at the wrong end of the division.
Ward, back from suspension after being sent off in the play-off final in May, broke the deadlock after wide-man Keith Treacy found space on the left, producing a low cross as the little striker raced in to finish with a right foot shot wide of the helpless Scott Loach.
Evans saw his angled shot from 20 yards graze the foot of the far post before ex-Watford front-man Darius Henderson headed narrowly over after Evans' speedy feet had carved out the opening.
Centre-back Matt Kilgallon squandered an opportunity as United, in total control, failed to hammer home their advantage.
Watford midfielder Scott Severin fired over before Evans killed off Malky Mackay's side, running onto Treacy's pass before weaving his way past two defenders to finish clinically from 15 yards.