I've never seen him play CB, nor do I think he'd be much good there. Yes, he is tall, but he's a CM who has been converted to a holding midfielder in his 20's, he's not a natural defender at all. (also, you'd lose a lot of his best attributes by playing him there) He might be able to do a job there in an emergency, like Yaya did for Barca in the CL, but I wouldn't play him there unless there was literally no other options. Given how quickly he picked up the holding midfield role, you might be able to teach him to play CB, but I think that would be a complete waste of his talents if I'm honest.