I think a lot of the hyperbole is down to the fact we've been so starved of an actual proper central midfielder for so long. So it's no wonder we get giddy when we see someone with a bit of skill on the ball also who likes to make a tackle. This time next summer it will be 10 years since we signed Carrick and Hererra currently looks like the first new signing since then who comes even close to being as good as him. Which is amazing, really.
So far in this thread people seem to have only looked at clubs above us to make comparisons, for some reason. I've absolutely no doubt that the likes of Henderson, Schneiderlin, McCarthy or Mason would be getting insanely hyped on here if they'd been playing for United this season.
Not to be nitpicky, but no way will Henderson or Mason get hyped up on here, especially with their limited skill set and approach. Mason, especially, would have been strongly attacked by our fans, for sure, given how limited he is in his game.
I do agree that there is some exaggeration going on with regards to Herrera. I'm guilty of this as well a few times. As you said, it most likely has to do with the fact that we haven't signed/played such intelligent and skilled midfielders in a while. However, given how Bayern's players improved in their second season under van Gaal compared to their first (particularly Schweini and Gomez), I'm sure Herrera will also improve and become more influential than he currently is.
I agree he's got quite a nice skillset, not sure if he's the "most complete", that part seems more to attract debate than anything else. But yeah, a really valuable player in the team. Similar to Blind, in Van Gaal's philosophy, they are really vital, their intelligence on and off the ball brings the best out of the whole team.
Having said that I do think he should not be given the creative burden in the team. Which is why having someone as creative as Mata to play next to helps him so much. He's perfect in keeping the ball in the middle of the park and driving it forward, but he always looks for Juan (or Valencia if Juan is marked) to play the final ball. I am not sure whether that is due to him not being confident enough to play the final ball or if that is just his style of play, to keep it short and pass and move. In any case, it works really well and his chemistry with Mata is fantastic. There's no doubt that this partnership has to be retained going forward.
This is most definitely his style of play for much of his career, but I wouldn't say that he lacks the confidence to play the final ball. Mata's first goal against Liverpool away is an excellent example of his great final ball, but with Fellaini being the most attacking midfielder out of the 3, Herrera is most likely playing a more reserved role, particularly as Mata isn't a proper winger and cuts inside quite often. This setup is quite similar to how he set up Bayern in their first season where Robben and Ribery created all of the chances (Young and Mata), Muller was more of a goalscoring player behind the striker (Fellaini), and Schweini was more reserved than his time as a left wide attacking midfielder (Herrera).
Even with Bilbao, when he wasn't an attacking midfielder, he was not that risky and didn't get as many assists as, say, Fabregas. However, like Cazorla, he dictated Bilbao's midfield as a #8 under Bielsa and Valverde, particularly for Bielsa, and also as a #10 as a Zaragoza player and under Valverde. He can definitely play nice final passes, but he needs to be given the freedom to do so. However, even in a more reserved role, he's very good at moving the ball around, maintaining possession, and creating gaps. He just needs to improve his passing range and defensive positioning to be complete.
I agree to some extent. The reason we don't see as many of those passes is because he plays further forward therefore the range of passing is limited and it generally has to be short, incisive passess. You don't really see him as often as Carrick/Scholes where they literally take the ball of the CB's feet and then have the whole game infront of them, from this position a longer defence splitting ball is more natural. Herrera mainly receives the ball in the opponent's half with his back to goal which make it more difficult to show his expansive passing game if it does exist.
However there has been times where balls have been on to the wing where it has needed a 30-40 yard pass and he's chosen the easier option or he's tried and it hasn't come off. I'm not sure whether he doesn't have the confidence/ability to try it or whether LVG has coached him out of it.
The longer passing isn't a part of his game as well. Under Bilbao, especially under Bielsa, he would always have short-passing options around, and the team's attacking, dynamic approach meant that long passing wasn't really necessary from Herrera, particularly when he had Martinez/Benat/San Jose/Iturraspe doing it. However, under van Gaal, I do believe that he'll add the long passing to his game, and he's already attempted some long passes in the last few matches.