Only a stunning double save from David Raya in added time denied United victory on Sunday, the Arsenal goalkeeper flinging himself in front of Bruno Fernandes from close range and then leaping to his feet to claw the spinning ball away just before it crossed the line.
Thousands of United supporters had marched in unity to protest the club's ownership before kick-off.
But on the field, Ruben Amorim's team produced a unified display of their own to deal another blow to Arsenal's flagging title challenge.
"Today the efficiency we had in the last 20 metres wasn't good enough," Gunners boss Mikel Arteta said.
"We know that because for big parts, a lot of things that the game demands when you come to Old Trafford, we did superb.
"But at the end you have to capitalise on that when the team is so dominant and much better than opposition, and today we didn't."
Fernandes fired home a free kick from around 20 yards in first-half added time and only a slew of missed chances stopped United from extending their lead after the break, before Arsenal levelled through Declan Rice in the 74th.
"What I can say is we need more Brunos, that is clear," United boss Amorim said.
"Not just the quality and the character, he makes some mistakes, but in this league the availability is so important and he is so decisive with and without the ball."
Elsewhere, Chelsea moved up to fourth after a 1-0 win against Leicester City plunged the Midlands club into further strife near the bottom, and Spurs fought back to earn a 2-2 home draw with Bournemouth.
Marc Cucurella struck Chelsea's 60th minute winner at Stamford Bridge after Cole Palmer's first-half penalty was saved.
Victory boosted Chelsea's chances of a return to the Champions League next season, moving them ahead of four-time defending champions Manchester City, who'd lost 1-0 to Nottingham Forest a day earlier.
Enzo Maresca's team took full advantage of City's latest loss as Cucurella's long-range effort was drilled low into the corner to beat Leicester goalkeeper Mads Hermansen.
Earlier, Hermansen had been the visitors' hero when diving low to save Palmer's penalty in the 22nd.
It was the first time Palmer had failed to score from the spot in the league in 13 attempts, his 100 per cent record from his previous 12 having been a league record.
The 2016 champions Leicester are now six points adrift of safety in the relegation zone.
At Tottenham Hotspur Stadium, Son Heung-min's late penalty completed an unlikely comeback for Ange Postecoglou's men after Champions League-chasing Bournemouth had dominated the chances.
Marcus Tavernier gave the visitors the lead in the 42nd minute and Evanilson doubled the advantage in the 65th, while Justin Kluivert also hit the post.
Pape Sarr's mis-hit cross then caught out Bournemouth goalkeeper Kepa Arrizabalaga and flew into the back of the net in the 67th minute to give Spurs hope.
And it was Arrizabalaga's error - bringing down Son in the box - that led to the equaliser from the penalty spot as Son stepped up to convert.
With PA.
