I would have Herrera as the DM which frees up Pogba & Keita. Keita is very versatile, he's only 22. He can do any role in the midfield(same as Herrera).
Signing Keita from RB automatically makes Herrera the first choice DM. If Jose asks Herrera to stay in front of the defenders and not move an inch after winning the ball, he would do just that. He's very disciplined.
Keita can play as a DM too. He played 17 league games there last season. He's tenacious in tackling and his positional sense is near world class. Dier gives us nothing. I'd rather have Lee Cattermole in that role than Eric Dier. I saw all the 5 games Dier featured in last season's UCL....trust me, he's out of his depth!
Forget Keita mate that's not going to happen. Well it had better not happen is more what I would want to say