Solskjaer
Van Gaal
Moyes
Mourinho
Solskjaer's issues are all tactical, and I have no issue with anyone thinking he's well out of his depth at this level, but he's definitely left a more solid platform than any other United manager has since Ferguson. The club seemed absolutely toxic when Mourinho left, the squad was pulling in several different directions, he'd alienated many players and so on.
The job is better now than at any other times Moyes was appointed, by a long way too. The next manager is walking into a dressing room with better characters, less deadwood, more depth in key areas, good young talent and solid players brought in. The next manager doesn't need an overhaul, they might just need one or two starters and that's all. I feel like a lot of the anger at Ole has been so vociferous that people have had to go beyond him just being "tactically inept" and start criticising the "shit squad", "awful signings" etc.. I think that's unfair, I think Ole did everything right at United off the pitch and even though that will never win you trophies, it might win the next guy trophies.
As I said though, Ole did a lot of the hard work off the pitch and the next manager has something solid to build off. No more radical changes in direction are necessary, just an improvement tactically may be enough for this squad to start winning things. Other managers were detrimental off the pitch, which is why United ended up with Schneiderlin, Schweinsteiger, Fellaini, Di Maria, Bailly, Falcao, Valdes, Mkhitaryan and so on, the alienation of Shaw, the treatment of Pogba. The squad now is in a much better place and Solskjaer deserves credit for that. Hopefully the next manager can add a couple, improve tactically and take this squad to winning trophies.