This fight sounds right on to me. Nelson is on the way up with his demolishion of Struve, and Mir is falling after being crushed thoroughly by Carwin.
If Roy beat Mir, he would shot that he deserves a spot at the table. If Mir beat Roy, it would stop his descent through the rankings and set him up for a comeback... again

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I think Roy has a shot at it, but I would put Roy as a +450 underdog. Mir has good boxing, great BJJ, and Roy doesn't hit as hard as Carwin or Brock by any means.
Big Country vs. Big Nog would also be a good fight for the same reasons as Roy vs. Mir would be good. It tests Roy to see if he's ready for the creme of the crop, or it sends Big Nog back towards the top.