In a 3 man midfield, if there is a creative, attacking player like Pogba, and a DM like Matic, the 3 rd mid need not be super creative nor ultra defensive.
What he needs to be is to provide cover for Pogba's forays forward, help Matic shore up defensive duties, use his high energy to press and hassle opponents, carry the ball and sometimes connect the attack with the defence. A high energy, strong box to box presence is needed and I think Ander Herrera is perfect for the role. Last year, he registered 7 assists for the season, quite good for a box to box mid.
That being said, I think his passing has been too conservative at times, and there is the fact that he is not good enough to replace either Pogba or Matic when one is missing. But IMO, he is an excellent option in a midfield 3 with Pogba and Matic.
It is near impossible to replace Pogba(don't think there is a mid now who can do everything that Pogba does to a competitive level, maybe SMS), but in Jorginho, we will find a mid, who can replace Matic whilst still providing a more creative spark from the deep.