I was just looking at our stats again, and it's really clear that Pogba's performances (and the whole team's performances) suddenly dropped off dramatically around mid-February when Herrera got injured. There's probably a few things going on (maybe tiredness, etc.), but it seems pretty clear that without Herrera our midfield went to shit.
Ole's great start relied almost entirely on a midfield three of Matic - Herrera - Pogba. That's definitely a midfield that can be improved on in terms of quality, but in terms of the players' functions it works pretty well and is a pretty classic set-up in the 4-3-3 era. Matic is defensive screen (and not too mobile), Herrera is the runabouty water-carrier, and Pogba does the creativity and is more focused on attack. It's a bit like a shittier version of the Pirlo - Vidal - Pogba combo that worked so well for Juve. None of McT, Fred, or Pereira could replace Herrera (though they maybe do a better job replacing Matic). You can see that Matic and Pogba's possession and passing stats drop significantly once Herrera's gone.
We seriously need someone who can be the box-to-box runabouty midfielder, someone like a young Arturo Vidal or a Keano. To say nothing of improving on Matic.
If Pogba leaves this summer, as far as I can tell, we basically have no midfield. It would be a total disaster.