Maybe it's an arrogance that they think previously disruptive players will toe the line because they're playing for the mighty Manchester United.
As I understand it Rabiot's attitude has been better at Juve than it was at PSG and he's become a regular in the France side. For the reported £15m fee that could be a good deal but it doesn't feel like a dressing room though where you'd want to be bringing in a player that has shown in the past he can be disruptive.
Arnautovic is just desperation. Like a pub league manager being short the night before a game and scrabbling through his list of contacts for somebody that he can rope in for a game.
I think his reasoning is to get in someone he can trust, a strongman type(according to him), and someone who wouldn't back down for someone like Ronaldo? And possibly to re-affirm what he wants in the dressing room and on the pitch.
A "dressing room signing", if you will.
I can understand Arnautovic in that regard, but Rabiot? Yes, he would fall in line behind Arnautovic, but brining in an unhinged player into an already unhinged dressing room is asking for trouble!
You need to bring him into a "settled" dressing room.