Ole Gunnar Solskjaer’s side suffered their third loss in the Premier League from their opening six games and are now languishing in 15th place in the table.
The Gunners got their goal from the penalty spot through Pierre-Emerick Aubameyang in the second half after Paul Pogba clipped Hector Bellerin in the box.
The Gabon international struck home from 12 yards to end his run of five league games without a goal and condemned United to yet another home defeat.
Roy Keane laid into the Red Devils’ performance, insisting they have a complete absence of leaders, and also singled out Rashford’s ‘shocking’ demeanour.
‘I’m just not convinced with these players,’ Keane told Sky Sports.
‘We often talk about characters at Manchester United, I just don’t see enough of them.
‘I looked at Rashford closely today, he’s had a load of plaudits over the last few months, I know he’s still a young man but his body language today was shocking, really shocking. Shrugging his shoulders when times weren’t going his way.
‘You roll your sleeves up at Manchester United, when things aren’t going your way that’s when you look at your leaders and characters.
‘Really, really worried for United now. The last year, year and a half you’re saying give them the benefit of the doubt, recruitment, what’s happening upstairs, there’s problems.
‘But what I saw today going back to the home games even against Palace, Brighton away, Spurs at home even Chelsea, real lack of quality at Manchester United and long, long way back for this club.’