He's not suffering from Appendicitus though. He had a perforated appendix and had it removed in an emergency operation. He then suffered from complications with his immune system and a bad reaction to the medication he was taking. Dier then had another operation in the summer related to all of this but exactly what it was is a bit of a mystery as it doesn't seem like the player or club have revealed exactly what it was.
It's possible he's still taking medication related to the ongoing immune issues. Lots of medications cause weight gain, but that's purely speculation.
Personally I don't think he's fat, I think he just put on too much muscle and it's affected his mobility. But it seems like he might have trimmed up a little bit recently. Perhaps a new training/diet plan.