I think Man United will be ok. I have seen both matches and in the second half vs Brighton, I thought they were quite good.
But and its and important thing to consider, there is a real need to do everything possible in the next couple of weeks to get a decent striker in. Generally speaking the teams that end up in a relegation battle, well all the pundits say the teams most at risk are the ones without a reliable goal scorer. Currently that applies to Man Utd. Martial has never been consistent and is often injured, Rashford seems to have lost all confidence, Sancho has not really been able to make the step-up from the Bundesleague to England (not sure why) and even the great Ronaldo seems to have found one opponent he can't beat: time.
I understand the issues with Arnautovic, but its a bit alarming that ETH wanted him (on a football ability level, not the other stuff), he does seem to only want to transfer in people he has worked with previously or Dutch - or both. Holland does have a rich history of producing good sides and players, but its also a small league/a small country so perhaps he needs to be more open minded especially in these most worrying times.
On the newbie section there is a thread discussing a sneaky move to gazump Aubameyang from under the noses of Chelsea (well they are trying to do the same with United's number 1 target of this summer Frenkie de Jong, so its fair). The trouble is, Aubameyang will really be wanting Champions League at his age (33) which he can't get at Old Trafford, and a massive wage offer will probably not change his mind on that (Chelsea wont be paying him pennies). Players at the top for ten+ years are so super rich when they get to their early 30s, they can prioritise glory over money more so than in earlier times. Finally would United want another troublesome ego on the staff (& Aubameyang certainly can be that) as they have so many of those already.