I do like Mbemba and it's probably harsh to say Rafa has written him off, it's just Clark and Lascelles have formed a pretty solid partnership at the back and it would be daft to keep swapping CB's every other game. At the moment, Lascelles would be the one that would have to drop out if Mbemba came in and even though Rafa left him out at the start of the season, I can't see him dropping our captain for an extended period when he hasn't made that many glaring errors.
Good deal for both teams and both players imo.