Quite simple really. Last season, we had no real DM. So Herrera was tasked with that role and he looked better than what we'd seen before (Schmidfielders and Fellaini) plus he looked like he was trying. Positionally, he was all over the park - but nobody noticed as we generally had Carrick cleaning up behind him - apart from having Rojo, Bailly and Jones having standout seasons. This season, Matic is the DM and has instantly looked so much classier. He's enabled us to play 2 in the middle and look good - something Herrera just could not do.
Last season, people blamed Pogba for being unable to play in a midfield 2 - but that excuse doesn't stand up when you see how good a midfield 2 looks with Matic as Pogba's partner. Still, people's love for Herrera means that few attribute this to the big step-up that Matic has been over him. Now, we're seeing Herrera tasked with being more attacking in the absence of Pogba and Fellaini - something that a lot of folks on here wanted for the last 2 seasons. We're finally seeing why no manager agreed with this view though. He's simply not good enough. His positional discipline is atrocious and he's all over the place. Add to this his propensity to misplace balls and play too soon and Matic is always stretching to cover the space. Some see his "all over the place" displays as "dynamic" and say "at least he's trying", but dispassionately, that's all the stuff that Henderson does too and we all know he's cack. In a 4-2-3-1, both of the midfield 2 need to be disciplined in position. Else, it all starts falling apart - as we saw against Liverpool and then Huddersfield.
In summary, Herrera looked excellent in a team where top-4 was the aim and we didn't have a Matic. With heightened expectations and a much better DM in place, he's looking like what he is - an above-average player with great attitude and heart, but one that doesn't fit into a squad with major ambitions. I'm not even sure he'd be a great Matic replacement, except in the early rounds of Cups - given how he's failed to work in a midfield two since, well, forever. If we need to play 3 in the midfield to accommodate him, it means we buy - and if so, I'd rather we bought someone who's closer to Matic's level.
About time we accepted that Mata, Herrera, Young, Lingard, Darmian (and perhaps even Mkhi if this is his level) are just not good enough any more and we need upgrades.