Casey just played an 82 game season and was out of gas at the end. Maybe they feel it is better for him to rest a bit and then work on his strength.
Good excuse. If they think he couldn't work on his on-ice game decisions, or perhaps get some confidence to springboard him into summer training, it's indicative of deeper problems than them thinking he was ready to begin with. The coddling has been one of the silliest aspects of Botterill's tenure. What he tried to do with Mittelstadt was akin to Regier forcing players into the lineup early... and none of them have hit the levels expected at their draft. Pardon me if I don't endorse anything that these guys think is good for anyone after watching two years of inept actions.