The Celtics can waste money as they see fit, but guaranteeing that contract with the belief that it benefits the locker room is a failed premise.
If Boston does this, it must believe its players are stupid. So the Celtics give Rivers an extension this summer, and thus, the players say to themselves, "Now we have to play hard for him, have to listen, because we're sure he'll be around for the long term?"
Who's kidding whom here? Boston knows that another bad season gets Rivers' extension blown up and gets it a new coach. As Houston Rockets coach Jeff Van Gundy has always said, "The only thing guaranteed is the money."