For Watford this was the thrilling story of another last-gasp triumph thanks to a 92nd-minute goal by Tom Cleverley following an equaliser from the penalty spot by Troy Deeney. For Arsenal it boiled down to a tale of two Germans and one 20-year-old Brazilian.

Per Mertesacker was on course to be the hero after heading Arsène Wenger’s team towards victory with a goal on his first league start for 18 months, but then Mesut Özil’s entrance coincided with the start of the Watford fightback, which was boosted by a spot-kick awarded following a sudden loss of verticality by Richarlison.

Wenger was appalled by that decision, adamant there had been no foul when Héctor Bellerín challenged the Brazilian, but even the visiting manager conceded Arsenal contributed to their own downfall. Mertesacker’s goal in the 39th minute gave Arsenal control of what had been an uneventful match and Özil could practically have confirmed victory by adding a second just after the hour.

Instead he shot feebly at Heurelho Gomes after being put through by Alex Iwobi. Punishment for Arsenal’s lapsed standards was meted out shortly thereafter, albeit starting with the dubious penalty.

Watford were not satisfied with a draw and having scored late goals this season against Liverpool, West Bromwich Albion and Swansea, they snatched maximum points after an attack that triggered panic in the Arsenal box. After a couple of shots ricocheted around the penalty area, Cleverley kept his cool to fire into the roof of the net from 10 yards. By then Arsenal had been deprived of two key players as their problems piled up, Danny Welbeck and Laurent Koscielny limping off with hamstring and calf problems respectively.

The departure of Welbeck, who had only just returned from injury, led to the introduction of Özil, who had begun on the bench as Wenger opted for Iwobi and the solidity that had yielded a 0-0 draw at Stamford Bridge last month. Their other away league games this season were the 1-0 loss at Stoke City and the 4-0 defeat at Liverpool in late August.

Alexis Sánchez and Aaron Ramsey, physically and mentally drained after their efforts that ended in World Cup elimination for Chile and Wales respectively, were not even included in the squad.

Wenger watched with a sense of vindication as his team enjoyed the better of a tight first half. Arsenal were energetic and disciplined and their midfield minimised the amount of pressure put on a defence in which Mertesacker was starting because of injury to Skhodran Mustafi.

Arsenal were the first to threaten. Adrian Mariappa made an excellent tackle to halt a run by Welbeck in the ninth minute and intervened again two minutes later to prevent Welbeck turning the ball into the net from close range after a volleyed cross by Sead Kolasinac.

Watford created their first chance in the 18th minute, when Abdoulaye Doucouré ventured into the space behind Bellerín and delivered a dangerous cross. That might have ben rewarded with a goal if Roberto Pereyra had not mistimed a header from seven yards.

Mohamed Elneny tried to bypass the Watford defence on the half-hour by going straight for goal from 25 yards but his shot flew over. Then Alexandre Lacazette teed up Welbeck with a sumptuous dink, but José Holebas was alert and lithe enough to poke the ball off the foot of the striker before he could shoot.

Arsenal were gaining the upper hand. They capitalised by scoring a goal of textbook simplicity. Granit Xhaka floated over a corner from the right and Mertesacker made Mariappa and Cleverley look Lilliputian as he ignored their challenges and headed into the net. By half-time Arsenal were looking comfortable.

Özil, with a fine pass, offered Iwobi a chance to put them even more at ease, but Gomes tipped away the shot from the Nigeria international. Then Iwobi presented Özil with a better chance but the German’s attempted finish lacked power and precision, allowing Gomes to save.

Worse, Arsenal were losing control, no longer able to keep Watford at bay. Richarlison may not have been fouled after bursting into the box but his raid was a sign of how play had changed course and Arsenal were losing their grip. Watford snatched victory after triggering panic in the away defence, Cleverley keeping cool to whack the ball into the net from 10 yards. The result took them up to fourth, overtaking not just the visitors but also Burnley, Liverpool and Chelsea.