Well you seem to be taking his treatment to heart in a way I suspect you wouldn't for other players. I don't know Koeman's exact thought process, but the most likely reason for his sporadic omission is tactical. Like Robbie Brady at Hull, he's being played in a position to which he isn't entirely suited, so when things aren't going right he will always be vulnerable to a more specialised player taking his place. It's difficult for a player when the formation doesn't allow him to play in his best role.