Can't Invite a Fighter UFC 4

I'm trying to learn the Celtic Cross. I have started 2 careers on UFC 4 and in both, the fighter who can teach the Celtic Cross is unavailable until you reach the UFC Top 15. So I've reached the Top 15 and in one of my careers, even became champion, but now it says they're unavailable because they are potential opponents. So, I've fought and beat him, and it still says I can't invite him to teach me the celtic cross due to him being a potential opponent. How do I make him available so I can learn the celtic cross?