Diego has been treading water for a long time. He wasn't on his way out like many of the guys BJ has beaten over the last ten years, but he hasn't improved a whole lot lately either, and seems to be jumping divisions desperately. He gets a lot of FOTN honors but that's because he's playing the stand and wang game now.
It's not that he just beat Diego, it's how he beat him, he did the same thing to Stevenson, Sherk, and Florian. His Lightweight title run is very underrated in my eyes, the level of dominance and the beatings he dished out to Joe Daddy and Diego will go down in history as some of the most brutal.
I hope that win/lose/draw that he does go back down and try to make one more run at Lightweight, he's still a competitive guy at that weight and he might even be able to win the belt. I'd certainly take him over Ben Henderson. Edgar was just a bad match up for him.