With one minute remaining of normal time, the Red Devils were 1-0 up and cruising thanks to Juan Mata's 68th-minute strike.
Arsenal were awful at the Theatre of Dreams and an equaliser seemed extremely unlikely.
And then it happened. From Alex Oxlade-Chamberlain's dangerous cross, Olivier Giroud lept highest to power his header past David de Gea and secure a point.
United's players will have inevitably felt hard done by, with Phil Jones admitting the draw felt like they had been "slapped 6-0".
"It feels like we have been slapped 6-0," he said, per Sky Sports. "We completely dominated the game from start to finish.
"I thought we defended well, created chances and it was just one of those things. They got one chance and scored.
"We have to pick ourselves up, we are pleased with the performance but not with the result."
Jose Mourinho had every right to be happy with his side's performance - after all, they largely contained an Arsenal outfit who are unbeaten in 11 Premier League games.
But for Jamie Redknapp, speaking in the post-match analysis on Sky Sports, he was deeply disappointed with one player and blamed him for the draw: Marcus Rashford.
The 18-year-old allowed Oxlade-Chamberlain to beat him far too easily down the right-hand side and cross to Giroud, who was always going to win the header.
"I'm a big fan of Marcus Rashford," Redknapp began, "but I don't think he does well enough at all. Whether you're a centre forward or not, you've got to do better.
"He let his team down there. He doesn't get out to Oxlade-Chamberlain quick enough, he lets him run at him and pick up pace. He gets beaten miles too easily.
"You're hanging on for dear life to win such an important game. He didn't do enough, he really didn't, and I'll be amazed if Mourinho isn't telling him that in the dressing room."
Ironically, Mourinho picked Rashford ahead of Wayne Rooney for his pace, which failed to pay dividends when closing down the equally-fast Oxlade-Chamberlain.
"I thought we were going to have the ball," Mourinho said, per ESPN. "Arsenal are a team who let their opponents play.
"I thought we would have space and the ball would arrive quite easily to the attacking players.
"I believed ones like [Juan] Mata, [Anthony] Martial and Rashford were faster than Wayne, better attacking opponents one to one in the last line. I thought it was the best option."