Well his final season (2009-10), we lost in the first round. The next season, without him, we lost in the first round. We then missed the playoffs the next two years. I would argue, though, that dropoff was caused by losing Brandon Roy for most of the 10-11 season (and permanently after that), not an assistant coach.
Much like I would argue the turnaround in PHX has been because of CP3, and not the coach.