Back at the other end Manning flashed a fizzing attempt past the post, but Watford remained in the ascendency and Hassane Kamara was unfortunate to see his close-range effort denied after finding himself through on goal.
Mattie Pollock was the next Watford player to come on for his league debut in yellow, alongside striker Vakoun Bayo, before home appeals for a spot-kick were unfounded when Sarr was felled by Manning.
The game was temporarily stopped due to a fault with the officials’ communication system, and once restarted Swansea went on the attack, with Joël Piroe forcing Bachmann into a smart stop.
Seven minutes into the 17 added on to the normal 90 Swansea took the lead, with a teasing Manning free-kick finding Cabango in space, who stayed composed to head past Bachmann and make it 2-1.
The Vicarage Road faithful were dismayed when match official Tim Robinson refused to award a spot-kick for what the Hornets' players believed to be a handball with the clock deep into the red, and the final whistle soon sounded to condemn Watford to their third defeat of the campaign.