While I absolutely hate how Arsenal gave 2 free goals, (I also don't think it's acceptable compared to their performance so far); I wouldn't blame it all on "sloppy defense".
I agree there was more at play here, but their defense
was atrocious. Look at United's second goal (
https://streamable.com/ho7f8 starting around 2 minutes). Arsenal's defending looked like when you press the "call another defender" button in Fifa or PES. 4 of them just running blindly at the ball, doing nothing useful, and Kolasinac ends up passing it straight to Lingard. And yeah, yet again De Gea is the only thing saving United from certain defeat.
they are 8 points adrift of the 4th place ranking.
Before the start of the season they were title contenders. Within a few weeks that changed to an easy top 4 finish. Now they are looking like they will struggle for a top 6 spot. I think Mou will be sacked when they are inevitably knocked out of the Champion's League.