His playstyle seemingly changes every season. Or the different roles of different managers affect him that much to a point of losing some of his traits.
LVG,
1st season: Playmaker + Attacking midfielder + Holding midfielder (one of our best players of the season).
2nd season: N/A (Out of squad - don't know why).
Mourinho,
3rd season: Holding midfielder + B2B + Defensive midfielder (lost that creativity and finishing skills, weird, one of our best players of the season).
4th season: N/A (Barely playing, injuries, bad forms and Matic).
2nd season syndrome per manager?
I seriously thinks he's carrying hampering-niggling injuries, which he may got from last season and worsened during the pre-season, but is not serious enough to stop making him available. Mourinho and him just doesn't come out and use it as an excuse. Herrera strike me as a player that doesn't mind playing with injuries. Still, should've played better when given the chances.