Antonio Valencia and Jesse Lingard both scored off brutal Arsenal turnovers to give Jose Mourinho's side a dream start.
And there were plenty of emotional reactions on social media as the Gunners shot themselves in the foot twice in the early going.
Just woke up, is Arsenal playing?— Alexi Lalas (@AlexiLalas) December 2, 2017
FYI @Arsenal the game started 10 minutes ago. You might want to remind the players.— Deluded Brendan (@DeludedBrendan) December 2, 2017
Imagine giving the ball away in your own half. And then doing it again 5 minutes later. Welcome to Arsenal.— FPL Guidance (@tips_fpl) December 2, 2017
Arsenal are self-destructing here, but Manchester United have been devastating in taking advantage of the Gunners horrendous errors.— Gary Lineker (@GaryLineker) December 2, 2017
Mourinho has parked the bus.....in the Arsenal-goal— Jan Aage Fjortoft (@JanAageFjortoft) December 2, 2017
21 - Antonio Valencia has never lost in any of the 21 previous Premier League games in which he has scored. Omen?— OptaJoe (@OptaJoe) December 2, 2017
Mourinho is driving the bus all over us, then reversing it back & forth over our shattered carcasses🙈🙈🙈🙈🙈🙈. #afcvmufc— Piers Morgan (@piersmorgan) December 2, 2017
What a filthy assist by Martial, that was absolutely class.— Devils of United (@DevilsOfUnited) December 2, 2017
Wenger on Lacazette: "He will play because he recovered quite surprisingly. In practice yesterday he played alright."— Chris Wheatley (@ChrisWheatley_) December 2, 2017
Arsenal look awful. Time for Wenger to go. Only one man can save them now. pic.twitter.com/q3veAuSUVJ— Paddy Power (@paddypower) December 2, 2017