Mikel played his part for Michael O’Neill’s men in the match but his effort was not enough to help them avoid defeat as Senegal international Sarr emerged as the hero of the match.
Sarr, who was handed his seventh appearance for Watford, scored the goal that made the difference in the encounter.
Stoke started the game on an impressive note, opening the scoring as early as the second minute through Steven Fletcher.
Tom Cleverley levelled proceedings for the Hornets moments before the half-hour mark with a fine finish.
Watford continued to push for a home victory and were rewarded for their efforts when Joao Pedro converted from the penalty spot in the 61st minute.
Mikel was cautioned in the 79th minute after a late challenge and two minutes later, Nick Powell levelled matters for Watford.
Sarr then scored the all-important goal that handed Watford victory in additional time when he fired his effort into the bottom corner after receiving a sumptuous assist from Ken Sema.
The strike is the winger’s second in the current campaign after opening his season account against Wycombe Wanderers in October.
Sarr featured for the duration of the game while his teammate and Nigeria international William Troost-Ekong was an unused substitute.