Again, how exactly does this affect the manager. The managers still were able to sign who they were appropriately able to sign. Sanchez was Mourinho's transfer, because he failed doesn't mean that wasn't true. VDB and Pogba are the only two transfers that I can definitively say were not the decision of, but were approved by the manager. Despite not have a sporting director, we signed players.
I'm not saying these decisions haven't affected Man United the club. But those structures are not affecting the managers ability to coach the team, or recruit players they ask for. Which is why managers we've had can't be said to have been dealing with structural failures when they've actually been the benefactors. They've been given space and time to manage, have been blessed with transfer kitty's no other club is giving outside of City, have been backed against players and yet have failed.
Of course they affect the managers.
The managers shouldn't be given carte-blanche to sign whoever they want, and considering we've only just appointed a DoF (who doesn't even seem to be fully doing the typical DoF job), then who exactly was signing the other players? Multiple managers have gone on record saying they didn't get the players they wanted.
The fact is that the managers have been let down by the lack of structure. Moyes came in and the lack of structure allowed him to gut the backroom staff and bring his own people in, as well as bin the transfer targets the club had in favour of spending all summer chasing Fellaini and Baines.
LvG came in and the lack of structure saw yet another complete overhaul of staff, but also an overly ruthless gutting of the squad alongside a host of scattergun signings and a completely new style of football.
Mourinho then came in, inherited a squad made up of players signed by three different managers (and Luke Shaw and Ander Herrera who were signed when we didn't even have one) and brought with him another massive shift in style of football. Masses of money were spent on ill-suited signings, one of which we swapped for another terrible signings after a year and a half, all while Mourinho publicly complained about a) not being able to get rid of players he no longer wanted and b) not being able to sign players he did want.
Solskjaer was brought in on a temporary basis, bringing yet another lurch in style, then the obvious lack of planning saw him given the job permanently, and it's since been revealed that he also wasn't allowed to sign his own targets, and that he was limited to just three or four senior signings per season regardless of cost or the needs in the squad. It goes without saying that again, the squad has been assembled completely haphazardly under different managers.
Now it's Ten Hag, another shift in style, still no real direction despite the apparent appointments of a DoF and Technical Director, a mess of a squad, etcetera, etcetera.
These structures (or lack thereof) quite clearly affect a manager's ability to do his job properly. That's not to say we've made good appointments (which in itself is a structural issue) or that Ten Hag is indeed good enough, but to boil it down to "well they got to sign some players, didn't they?" ignores the glaring issues around recruitment and the knock on effects that has on the quality of the squad its suitability for the manager (who should at least be in the same ballpark as his predecessor when it comes to style of play). It doesn't matter how much a squad cost to assemble or what wages the players are on if they're not suited to the style of play or, in some cases, simply not good enough, which is why blindly waving in the general direction of our (ever dwindling) transfer budget is completely missing the point.
The managers haven't been "given space and time to manage" either. That's a mad argument. Moyes was gone within a season, LvG won us a cup but was binned for not achieving top four after just two, Mourinho was sacked before the halfway point of his third season, same for Solskjaer, and now Ten Hag's facing similar pressure before the halfway point of his second season.
Three of the four post-Fergie managers have been sacked mid-season, and that's looking very likely to become four from five this season. That's not "time and space to manage" at all. That's a P45 ready and waiting as soon as you slip.