It's hard to evaluate Hearns as a middleweight, since he jumped around the divisions at bit. He lost to Barkley but he beat Shuler and Roldan quite impressively. He lost to Hagler but there's no shame in that. Hopkins lost to Roy Jones Jr., and beat Felix Trinidad and a few others, not sure how good his opposition really was.
This is a fight that Hopkins COULD win, if he was willing to take chances, particularly late in the fight as Hearns tired and became more vulnerable. Hopkins had the defense, footwork, head movement, clinching, well-timed blows, and general cautiousness to go rounds and avoid Tommy's killer blows, and at least be competitive. The question though, is whether Hopkins WOULD win. I have a tendency to think that Tommy's height, reach, speed, timing, and power, and general ferocity and boxing ability would keep Hopkins so cautious that he would not take the risks he would need to take to get to Tommy, and he would lose a close decision, even if Hearns had a couple shaky moments in the fight, particularly late.