I don't think an '8' is on our transfer list.
I think we need:
2 full backs
1 defensive midfielder
1 striker
But...if we were to lose a player in that position, say we deem it time to move on from Gundogan because of his back issues or Bernardo might still want to leave (hope not). Then I wouldn't be surprised that we take a punt on a free transfer.
Bernardo (assuming he stays) and De Bruyne would still be our starters, nothing changes there and if things went tits up we'd still have Foden and Grealish that can play there.
Plus signing another midfielder that could play both '6' and '8' like De Jong (alright not really a DM, but can control the tempo of a match), gives us even more scope to rotate in there.
Not saying Pogba's the right fit but on a 'free', I'm just saying I wouldn't be surprised if we did it.
Personally I'd question whether he's good enough, he's shown nothing in 5 years at United. I'd only trust it on the basis that Pep probably believes he can get the best out of him.