It has become obvious in this year’s NBA playoffs that Tim Duncan is old. San Antonio’s showdown with Oklahoma City in the Western Conference semifinals on Thursday may be the final game of his career, but how much Duncan will play is a mystery. He’s now on the court less than ever, and when he is, he’s a 6-foot-11 shadow of himself.

But Duncan is indispensable even when he doesn’t play. In fact, he could spend the entirety of the Spurs’ elimination game knitting a sweater at the end of the bench, and he would still be the...