United edged out fifth-placed West Ham 1-0 thanks to Craig Dawson’s own goal to remain a point ahead of Leicester, who thrashed bottom-of-the-table Sheffield United 5-0.
On a costly afternoon for Spurs’ hopes of a top-four finish, Tottenham also lost Son Heung-min to a hamstring injury at the Emirates, while Lamela went from hero to villain for the visitors with a needless second-half red card.
Arsenal’s afternoon did not start well with captain Pierre-Emerick Aubameyang dropped to the bench for a breach of discipline, reportedly after turning up late for the squad’s pre-match meeting.
Lamela was introduced to replace Son early on and the Argentine opened the scoring in sensational style against the run of play as he wrapped his left foot around his right to fire into the bottom corner.
Emile Smith Rowe and Cedric Soares hit the woodwork either side of Lamela’s strike but the hosts finally got their reward for a dominant first-half performance when Martin Odegaard’s shot deflected in off Toby Alderweireld for his first Premier League goal.
“I think we played really bad in the first half,” said Spurs boss Jose Mourinho. “The 1-1 was not a fair reflection of the first half. We were poor, defending bad, no intensity, no pressing, some important players hiding.”
Alexandre Lacazette captained the Gunners in Aubameyang’s absence and also took the Gabon international’s duties from the penalty spot after the French striker was brought down by Davinson Sanchez inside the area.
Lacazette sent his international teammate Hugo Lloris the wrong way from the spot to claim a much-needed win to keep Arsenal’s slim hopes of finishing in the top four alive.
Lamela was given his marching orders 15 minutes from time for a second bookable offence.
Despite their numerical disadvantage, though, Tottenham’s best spell came with 10 men as Harry Kane had a goal ruled out for offside before the England captain hit the post and Sanchez’s follow-up effort was cleared off the line by Gabriel Magalhaes.
“The last 10 minutes was the worst we’ve played,” said Arteta. “We could not manage the game, we could not manage the tension.”
Arteta’s side are now 10 points behind fourth-placed Chelsea with a game in hand, while Spurs remain six points behind the Blues in seventh.
More Man Utd misery for Moyes
West Ham missed the chance to move into the top four as former Man Utd boss David Moyes remains winless in 15 visits to Old Trafford as the opposition manager.
The Hammers’ conservative approach was punished by one moment of misfortune eight minutes into the second half as Dawson turned a Bruno Fernandes corner into his own net.
Mason Greenwood also hit the post either side of half-time for United, who close the gap on runaway league leaders Manchester City to 14 points.
“It was a big weekend for us,” said Solskjaer as Chelsea and Everton also lost ground in the top-four race on Saturday.
“This is a big three points because we know West Ham are challenging to get into the top four.”
Five star Foxes
Kelechi Iheanacho scored a hat-trick as Leicester showed no mercy against the Blades in their first match since manager Chris Wilder departed on Saturday.
Iheanacho opened the floodgates seven minutes before half-time with a close-range finish from Jamie Vardy’s pass after a flowing move.
Ayoze Perez’s fine strike from the edge of the area 26 minutes from time doubled the Foxes’ lead before Iheanacho produced two more fine finishes to take his tally to five goals in three games.
Ethan Ampadu’s own goal sealed the rout for Brendan Rodgers’ side, who now enjoy an eight-point cushion over West Ham.
In the battle to beat the drop, Brighton edged three points clear of third-bottom Fulham with a game in hand by winning a frenetic south-coast derby 2-1 over Southampton thanks to goals from Lewis Dunk and Leandro Trossard.