It would certainly be a mismatch. Hearns's physical advantages makes it very difficult for me to see how Whikater can gain any traction in this match up. Whitaker is a great fighter, I rate him above Hearns in p4p lists. But this is a tall order (no pun intended) Pernell. I don't think Hearns would pitch a complete shutout, I think somewhere along the lines Sweet Pea would pick up 1 or 2 rounds.
This would be quite a fun and interesting fight, IMO. Just to see how a prime WW Whitaker would cope with such a freak of nature.
Hearns may have been at physical peak, but the only real top-level guys he had fought by the time he left 147 were Cuevas and Leonard - and of course he had been KO'd in one of those two fights. I'm not saying he was green, but he can't have been as wily as he was after adding the experience of fighting at lightmiddle, including the wins over Benitez and Duran. However, I do agree that Hearns would win clearly.
Cuevas was arguably his most impressive performance, along with Duran. Against Leonard, I don't think the weight they fought at played into it at all, but rather the fighter he was facing. I don't think handily out-boxing a guy like Leonard, only to be stopped in the 14th round, is much of a demerit for Hearns, especially in a matchup with Whitaker.
Hard to see how Sweet Pea takes more than maybe 2 or 3 rounds in this one, assuming it goes the distance. I don't think that Hearns will be mentally discombobulated by Whitaker dodging him, as he did a great job of keeping it together in several notable bouts, like a very young Hearns systematically crushing Cuevas rather than rushing in and leaving himself open when he first hurt Pipino. And as others have brought up, Hearns put on a great showing against Benitez in a fight where he was being forced to miss a lot and fighting with a broken hand against one of boxing's great talents. Short of Pea doing something like pulling down Hearns' trunks as he did to Roger Mayweather, I doubt that he's going to get into Hearns' head. One issue I found interesting was la-califa's point about how Hearns' right might go sailing over Pea's head, as it certainly would leave Tommy open to counters, but I have to think that Tommy would correct it in short order, although changing his usual punching method might throw him off slightly. Still, trying to argue a boxer's case based on the idea that he'll be too short for his opponent to land reeks of desperation and trying to find a way for Pea to have a chance. In the end, Pernell is badly outgunned here. As great as Sweet Pea is, I don't think he'd be doing well in this fight.Either Hearns cruises to victory or Sweet Pea goes down for the count for the first time ever.