Exactly. Halfway last season, around the Barcelona games, Ten Hag was really getting somewhere with United, and individual players. And he got there through his own doing: he wasn't inheriting Ole's tactics and trying to hang on to them. But somehow, things went downhill from there, and now especially this season.
It doesn't make sense that Ten Hag both was responsible for that uphill trajectory and is also an idiot who knows nothing about tactics and player development. But United's squad is arguably stronger right now, and Ten Hag if anything should know it better, and the PL as well.
So what changed? I would argue that the principal issue is that Ten Hag is having to adjust to playing a second-string defence of players that don't fit proactive high-line defending, therefore had to adjust his approach - and in the process messes up all aspects of his intended defensive and attacking play and patterns.
That's obviously on Ten Hag. Well, he can't help that United's squad is still so disjointed in its depth, that's due to United flipflopping on style for a decade now. But he probably could have managed the current situation better from a tactical viewpoint. That's not a reason to fire him though. I would say that he needs to be given time to train and play games with his intended first team. If things still suck then, then yes, time to go. But sacking Ten Hag because he isn't handling an absurd injury crisis well with this disjointed squad doesn't sound sensible to me.