Derek Fisher is going to bat for his former player J.R. Smith -- telling TMZ Sports the Cavs should NOT quit on the shooting guard after his brain fart during Game 1 of the NBA Finals.
There's a been a call from fans and NBA experts to bench Smith -- most famously from Stephen A. Smith -- who says J.R. just ain't playing good enough and needs to be benched for the rest of the series.
But Fisher ain't buyin' it ... explaining, "We're really only talking about one play.