I would definitely disagree with this. Sure maybe the Xs and Os tactics are worth only a couple of wins, but coaching has much more to do with developing players, putting them in the best positions to succeed and getting everyone on the same page. Just look at the Spurs as the classic example of almost perfect coaching. Boris Diaw and Bellinelli were playing like crap before they came to the Spurs, you telling me having guys turn around their career like that is only worth 5 wins?
That's the first thing I thought when I heard the statement that coaching only has an effect of +/- 5 games. Teams like the Spurs are built off the system and the culture there. If you put that same roster in a different atmosphere with a different coach, no way they'd survive this long consistently being one of the best teams in the league.
