I think it's been brewing for a long time & personally I don't think Ronaldo can be blamed so much for Man Utd's pains in trying to transition into pressing more when he's pressing more than players much younger than him as an OAP.
I don't think Rashford's been managed well either - physically or mentally. He's been asked to play with injuries, he put off a big surgery because of Solskjaer & Southgate's asking him to for club/country, he's received a ton of racial abuse and abuse for combatting child hunger (weirdly). He looks a shadow of himself compared to when he's in form. And I think if he's been mismanaged... chances are other United players have also been mismanaged as well.
Too few United players look like they give a fuck when they're on the pitch. Which as a Liverpool fan, I'm okay with seeing. But if I were a United fan, I'd be really questioning why some of these supposedly great players look like they can't be arsed to perform in a United shirt.
You compare someone like Ronaldo to someone like Pogba and I think there's a pretty stark difference in mentality. Ronaldo's got shitloads of talent, but also an incredible work ethic and attitude - whenever he's on the pitch you never really doubt his commitment to getting a win (although sometimes this season it's led to some moments of petulance that shouldn't really be allowed, even for a player of his stature). Pogba's got shitloads of talent as well, as proven by his time at Juve and France, but I don't think he's got the right attitude to be a big player in a top side.
When Pogba's on the pitch you never know which Pogba's going to turn up - but 6/10 times, you're probably getting the Pogba who thinks it's his teammates job to do all the hard work while he fannies about on the pitch. When the going gets tough at France or when it did at Juve, I'd probably count on his teammates to put in the midfield shift necessary to get a win... not him. I'd certainly never count on him to be the sort of midfield general he was expected to have been, who can grab a match by the scruff of the neck and change things.
There's too many players at United right now that think "I'm at United, I've made it" rather than players who think "I'm at United, now is my chance to win things so I can say I've made it" imho. I do think Ronaldo was a luxury signing they didn't need to make and that's maybe a tactical problem for United now... but I don't think he's a problem in terms of the toxicity at the club right now.
Long may the toxicity continue, though.