Juan Diaz by this time an actual close decision. It could be a lot wider if Casamayor doesn't throw to the body.
Still Casamayor could either take a beating or give a beating. It's a fight where it has many different scenearios.
Sceneario 1 and most likely: It's a competitive fight in where Diaz lands to the body and fights better on the inside. Casamayor lands some really good shots here and there and roughs him up. However Casamayor isn't active enough to win.
Sceneario 2: Diaz, the much younger fighter, throws many many many punches overwhelming Casamayor not even allowing him to get off punches. Casamayor doesn't have the legs that he used to and can't get out of the way. Diaz stops him late or one sided UD.
Scenenario 3: Casamayor really roughs him up and is dead accurate. He uses the ring well and lands wicked 1-2's that splatter Diaz. Casamayor lands good shots to the body here and there that tire Diaz. When Diaz tries to get inside, Casamayor holds and uses his head a lot. Casamayor wide and surprising UD or late TKO.
Scenearo 4: Casamayor roughs and holds in a boring fight. Casamayor ends up cutting Diaz with an accidental headbutt in a fight that'll be ruled a NC or a TD. It could go either way here because some judges will like the punches thrown by Diaz or the roughness of Casamayor.
No way I see Diaz losing the decision at this point in Casamayor's career, but he could get caught coming in with a perfectly timed counter and knocked out. Casamayor still has more power and better timing. 65/35 chance of a Diaz win most likely. Who knows how he'll look against Marquez though, maybe he's back for real.