Admittedly I haven't watched that much of United this season, but he's played what, 10 games this season? And got himself sent off against City. Even if the rest were 'decent' games he's played some 80? games for United in his career and he's been below par. He's not a good tackler, his positioning isn't the best, he's not good at anticipating situations, his heading isn't good... He's a lower mid table defender, at best.
actually 40 of those games were very good, then he nosedived badly, and is now starting to recover some form