Van Gaal's desire to shoehorn three strikers into the first XI kept Herrera out of the team. Fellaini and Mata themselves have been victims of that, its harsh to blame them. Over the winter we saw good performances from Herrera, Fellaini and Mata, which were often rewarded with being put back on the bench so Falcao and Van Persie could play.
What you say about Herrera's performances being virtually unaltered I agree with though. Ander wouldn't have a goal, assist or key pass every 60 minutes to his name if he'd had a serious dip in form. Statistically, his output has remained strong all season. The only obvious alteration in his game is his positioning is more disciplined and his passing less ambitious. That's it. Those are minor tweaks to the game of a player who was always going to be huge for us.
Like Jones says, it was obvious three years ago that Ander Herrera was going to become a big player. Thank God its for us.