I think it could go two ways: Hearns operates behind that great jab and drops the righthand on occasion to keep Valdez honest and wins a decsion. OR Hearns builds an early lead but Valdez keeps coming and eventually stops Hearns. I'd pick the second outcome if i had to bet.
Rodrigo Valdes who stopped Bad Bennie Briscoe on May 25 1974 in Monte Carlo, Monaco in round 7 to win the vacant WBC World Middleweight Title would stop Thomas Hearns. Rodrigo, or Rocky as many of his opponents called him had a very dynamic right hand. That right hand dropped an iron chinned Carlos Monzon in round 2 of Carlos last title defense on July 30 1977, in Monaco, which Carlos won unanimously. Valdes had good stamina, and was a good counter puncher. When he was dropped by Monzon in their first fight on June 26 1976 in Monte Carlo, in round 14, he arose immediately. I see Hearns keeping him at bay for the first 7 rounds with his long left jab. But then he would make the mistake of trading with Rodrigo only to run into Valdes right hand, ending the contest for Valdes. The only weakness on Rocky is that he tended to suffer cuts around the eyes.
Lots of talk on here lately about underating/overrating fighters, and Valdez could fall into the former, on his best day was a match for almost any MW down the ages, and I include the elite, a perfectly balanced, and well proportioned boxer, who could do it all, box like a dream moving in and out, letting go hard accurate combos, that left a lasting effect on his opponents, or could stand and close quarter trade, banking on his resilience and rock solid chin, the first 3/4 rounds of his 2nd fight with BB he took some of the hardest body punches I have seen, Briscoe ( as was his want ) winged in cruel vicious rib bending shots, lesser fighters would have been looking for the exit, not Valdes , he took his punishment without a whimper, bit down on his gum shield and absorbed the best a noted body puncher like Bennie could offer, the rest you all know. Come the Monzon fight, and again Rocky took his lumps , but still fired back definitely against a MW who ranks at best 1st, at worst 4th, Carlos held no fears for Valdez, nor would any of the illustrious trio of, SRR/Greb/ or Hagler, he came to fight and didn't care who was coming out the opposite corner, a very tough, fearless and rugged boxer indeed. stay safe guys.
Or Hearns ends him like he did Shuler. But ofc, you will pump Valdez to the heavens since he fought the ever overrated Monzon.