I think the Real Plus Minus numbers don't reflect how well Grant Williams has played lately, and honestly he looks like a plus defender to me overall.
That said, while we're talking about RPM, I would like the astonishingly large number of people on here calling for the Celtics to go after Old Grant Williams -- i.e. PJ Tucker -- to look at how wretched Tucker has been this season. 
Playing over 30 minutes a game, Tucker has managed a rousing -5.01 RPM, which is fourth-worst (out of 456 players) in the entire league. No amount of intangibles can make up for that. The man is old and terrible. Yes, he used to be useful, but he is cooked now. I would not trade Grant for him straight up. The TPE has to be able to bring in someone better.