Average coach. A downgrade from SVG. Bad decisions by Detroit.
How was he average? Out of curiosity.
I've always liked him and wanted him to do well -- back since he was an assistant in Seattle and got his first NBA head coaching gig in Minnesota -- but I've never been overly impressed by him. He isn't a bad coach but he is not a good one either. Just a common middle of the pack coach.
I never felt his teams overachieved relative to their talent. Except for maybe this year and even then they bottomed out badly in the playoffs. He didn't make his teams better.
He took over that .500 Minny team and then they immediately fell out of playoff contention despite having a prime KG to work with.
He went to Toronto and took over a lousy team. He was overly indulgent to bad players like Bargnani. Struggled to get the most of role players like Amir Johnson, Ed Davis, Bayless, Barbosa. Then they got Valanciunas. Struggled to develop Valanciunas. Chose wrong defensive schemes in PnR defense (had Jonas stepping out on PnRs instead of hanging back like Hibbert) which hurt Valanciunas for years and hurt Toronto's team defense. Limited development offensively or defensively from Valanciunas in subsequent years. The team struggled more defensively than they should have in those years because of this reason. Regularly said he was going to get Jonas more touches offensively but failed to do so. Failed to reign in Rudy Gay's selfish play. Again, failed to deal with DeRozan's selfishness on offense. Never developed guys like Terrence Ross. Patrick Patterson is another player who came and went. Never improved. As most guys who play for Casey do not (improve).
He has always been a good defensive coach (very well regarded as an assistant coach defensively) but was a pretty basic offensive coach. His Minny teams and early Raptors teams were all dodgy offensive teams. His teams have all been lacking in ball movement. Overly reliant on one-on-one plays. Isolation plays. His teams do not move well off the ball. Do not pass the ball well. Basic XOs guy. His in-game coaching is unspectacular. He doesn't hurt his team too often and he doesn't help his team often either by making lineup changes / rotation decisions.
None of his teams have ever over-achieved in the playoffs.
Only 1 of his teams have over-achieved in the regular season (and even then, not by much).
In a 9-10 year career, it just isn't that impressive.