I don't think they'll have too many issues getting players off the books, it's quite clear some of the non-performance is a result of the club culture and poor tactics rather than individual ability.
There will be plenty of suitors, the problem will be wages and like what happened with Maguire and West Ham last summer, some players won't be keen to walk away from their current salaries that are setting up generations to come. There will be a handful that are looking to re-build their careers and will take the hit, I could see guys like Mount, Rashford and Dalot going to mid-tier European sides that are happy to gamble (we've seen it with McTominay and Sancho). Casemiro, Eriksen, Evans etc will end up in Saudi or retired, so there will be natural attrition anyway. It's the players in between that they need to be ruthless with - Bruno, Martinez, Onana, Shaw, Garnacho, Hojlund, Zirkzee etc. Some will naturally have to stay to ensure squad depth but there has to be a longer term plan to phase them out.
Even if they cant 'upgrade' them on paper with stars, they need to at least sign alternative PL quality players that suit the system and that don't carry the toxicity and laziness of the current squad. An example would be Schar on a free transfer to replace Martinez. He might be 34yo and only a stop gap for 12 months while looking for longer term options, but at least you know he's going to set standards in the dressing room, is a leader and will mentor the academy prospects. He's also not 5 foot 7. That's the sort of player Yoro can look up to.
I alluded to it in my previous post but as a club they will have to humble themselves in the market and look for bargain signings. There are plenty of players out there that would love the opportunity to play for United even without European comps.
I just can't see them doing it because the media and more importantly the fan base will tear them from pillar to post if they don't sign the big names.