Mertasacker made his first Premier League start for the Gunners in 532 days on Saturday, and the club captain headed Arsene Wenger’s side in front from a first-half corner.
The game turned in a matter of seconds in the second half, with Mesut Ozil missing a simple one-on-one chance before Watford won a penalty within seconds at the other end.
Richarlison appeared to go down under minimal pressure from Hector Bellerin, and Troy Deeney duly dispatched the resulting spot-kick to anger Wenger.
Watford weren’t done there as a goalmouth scramble resulted in Tom Cleverley finishing deep into injury time to send Marco Silva’s Hornets into raptures.
