I think Freddie Prinze Jr. did a bad movie about the Cape Cod League, and he could actually play.
Dennis Quaid in The Rookie was OK as well. He had stiffened a bit with age, but he did have a background as an actual player.
Some real players are role players or extras in Rookie of the Year as well.
Bernie Mac in Mr. 3000 wasn't a total hack.
But I think my vote has to go to Richard Pryor in Brewster's Millions.