I don't really see there being any argument for him being "dropped" other than after the Sheffield Wednesday game at the start of the season and even then he was back in to the team fairly quickly and being picked ahead of Bangura, beyond that it's clearly been normal rotation, and when both have been fit Engel has been the preferred option most of the time. You don't like Engel so you'll call it being dropped but when he's played as much as he is and often very quickly come back in to the starting lineup after a game on the bench there's not really much evidence to say he was simply dropped as if it was a result of a bad performance like the Sheffield Wednesday one.
It seems like a bit of a leap to use not being picked yesterday as an example of him being dropped rather rotated or rested considering he played the full 90 of the previous 5 games in which we haven't lost. I'd be more inclined to think it's a decision based more on Luke Thomas than any failing of Engel's.
Our defence has been pretty badly hit with injuries all season, I'm not sure there's even been much scope for rotation, but he's rotated the midfield a fair bit when we've had Hackney, Howson, Barlaser, and later O'Brien available, Silvera has been in and out of the team, more recently Greenwood and Azaz were out of the team for a few games and back yesterday, similar between Latte Lath and Coburn earlier in the season.