We do need a holding DM to get the best out of Pogba. The only midfielder in our squad that holds is Carrick and that's a problem.
It's why I think Fabinho would be a fantastic signing. He's 22, shown a lot of potential with brilliant physical attributes and a lot of time to improve further. He can play both RB and DM and that would be extremely useful given the relative uncertainty we suffer from in those positions with regards to the medium- to long-term.
I believe he does well in the 'tackling holding' DM role and has shown that in Monaco's games so far this season. In that sense you could compare him to William Carvalho, a brilliant tackler who breaks up opponents' play while holding a disciplined position to allow fellow CMs to move forward. Sure, he wouldn't be the best passer in that position but we wouldn't need him to be. He could really help us in that position and possibly develop more in it until we replace Carrick with another holding midfielder (like Julian Weigl).
And if Darmian doesn't work out (Valencia isn't a youngster anymore) then we would have a ready-made top quality RB already in the squad.
I don't see why we wouldn't pursue this signing. Hope we can get him after Monaco's qualifiers.