I think he's a little under-ranked here, but the guys above him are pretty solid too.
Last year may have been his best year yet. He took a lot of criticism for taking it easy on the players during the season, but that approach kept the team fresh and fueled their deep playoff run. Rivers' strength is as a motivator, and he's smart enough to cede tactical responsibility to his assistants. It'll be interesting to see how he responds to not having defensive ace Tom Thibodeau at his side quarterbacking the defense this season.
I think Doc will do just fine with Lawrence Frank backing him up in place of Thibodeau.