It occurs to me that this team is not so much physically tired, but mentally exhausted.
Mental focus in sports takes a lot of energy. Our players have had several long seasons with late playoff runs, never mind the summer Olympic and Team USA basketball tournaments some of the starters have participated in. Add in the fact that smart basketball decision making does not come easy for a few of our current players, especially the two All-Stars, J & J, along with the fact that I never got the sense that these guys have a natural hunger for competition.
Compare Tatum and Brown and some of the others with Celtic greats like Garnett, Bird, Cowens, Russell, Havlicek, McHale, Max, DJ, even Pierce. Those guys lived for playing basketball and competing and winning and kicking somebody's ass in the process. Larry Bird has always talked about how much he looked forward to getting to the Garden or even an away arena for game nights. Jayson and Jaylen seem to be out of gas mentally to me.