They've got five first team centre halves at the club. Not one of them is good enough for Manchester United. The youngsters they've got out on loan in Tuanzebe and Fosu-Mensah aren't even Premier League quality.
If we signed Chris Smalling, being a club who'll finish between 7th and 10th and looking to improve our options, that would be fine. He's at a club that is looking to win domestic and European trophies every single year. That is not fine. And he's probably the best of the bunch on balance. Jones is worse now than he was at Blackburn 7-8 years ago. Rojo is a joke footballer, I wouldn't want him on a free transfer. Lindelöf is weak, mentally and physically. Bailly always has a shocker of a game around the corner.
This is before we even consider the issues at full back, how ponderous and lacking in quality most of their midfield is, Pogba's attitude, getting any kind of consistency out of Martial and Rashford, Lukaku being a fat bastard, etc etc.