Hmm, as comes down to a few things, though the key word from your post is consistency. He has shown he's able to step it up, and played a very good game against Barcelona where he kept his discipline, it just hasn't been seen in any run of games without a gaffe. In terms of assertiveness, you want centre backs to step up and take charge of the situation - to me at least, he seems happy to just collect and hoof it up the pitch, or he doesn't use simple defending techniques and gets done for a foul, which more often than not results in a turnover of possession. Also his passing, for a passing side, isn't good enough. That said, he can turn it on now and again, I just feel as 4th choice, and still relatively young for a centre back, he needs to work on his positioning so he doesn't get done by tricky strikers so he becomes reliable. My theory is that is why he looks so bad on the wing and why so many fans think he's awful, as skillful wingers do him all the time as he's flat footed/on his ****.
His wages at 3m a year is are a bit steep, but that's a club problem and not the player, but agree he is needed until some of the younger centre backs get transitioned in.