For me, the real blame at Man U is Woodward. He has too much control as he's well in with the owners. The transfer strategy seems to be "make a headline grabbing deal", without actually seeing if its right for the club. Pogba was chased by Fergie because of his lousey attitude, but they break the world record to bring him back and back him more than managers who get sick of his attitude. De Gea has wanted to ge away for ages, but instead of selling, they keep him with a vastly improved contract because he was rated one of the best keepers in the world a few years back, even though his form is nowhere near that now. Martial is supposed to be moody and not bothered, but won't be sold because he's Joel Glazers favourite player.
To be honest, if Fergie was still there I think the same things would have happened and fans would be saying that he was past it and should have retired by now. Mourinho fell out with a few players there, so when he went they was a bit of a new manager bounce, but it ended up with Pogba and Martials egos running wild and a few flukey wins, particularly in the Champions League, papered over the cracks and got Solskjaer the job longer term, even though he's patently not upto it. At a lesser team with lesser players, he'd already have been sacked because they'd really be struggling, but there's still enough quality there for them to be better than most even when not playing well.