I can't imagine he'd come here to sit on the bench, because that's ptretty much what he'd be doing, assuming we signed Mount, too.
What we need is a player who can deputise for Casemiro, whilst also having the ability to play as an 8, especially when having to solidify our team/see out games.
As it stands, we have McTominay for that role, who I'd be happy with next season. Rabiot would also suit that role, but as I said, I can't imagine he'd be happy with back up. Other players who would suit that role could be Sangare and Neves.
That said, if he does come, then I think it'll be:
Bruno/Rabiot Mount/Eriksen
Casemiro/Rabiot
He'd be a player you can bring on to solidify the midfield but we could also start him in many games, whether it be the 6 or 8.
If not, then ten Hag will probably keep McTominay to fulfill that role.