Funnily enough Herrera is exactly the mould of a Mourinho midfielder. He's not small(considering Mourinho's used the likes of Makalele/Essien who are smaller players than him and Alonso is literally 1 cm taller than Herrera), he puts himself about and on top of that he has technique to boot. I think he provides balance alongside somebody like Fellaini as he can cover Fellaini as well as create chances when on the ball, while complimenting the physical nature of Fellaini.
Yeah he may not be black and bulky but he's exactly what Mourinho looks for in a midfielder imo.
I think Pogba's fast becoming a problem, but him alongside Ibra for me is where he'd play best.