From memory Rojo didn't really play at centreback very often in his first two seasons. It wasn't until Mourinho's first season that he got a good run there, and to be fair to him he actually did very well for three or four months (barring the two terrible challenges that he somehow didn't get a red card for). At the end of that season most on the Cafe voted him as our best defender that season, although that was as much due to everyone else also being very inconsistent as how good Rojo was. Unfortunately he did his cruciate to end that good run he had, and he's been constantly injured since then.
Don't get me wrong, it's obviously time to move him on. But it should be acknowledged that he did actually do quite well the only time he did play there consistently.
Actually I just looked at his stats and it looks like he did play centreback a fair bit in the first half of his first season. But even then it got broken up with two different injuries. Can't remember how he actually performed.