Hagler retired in 87, Jackson faced his first decent opponent in McCallum (1986) and was stopped in two rounds. There was no real time for this fight to be made. It could have been a classic as well.
Yes, Hagler was at the end of the rope in 87, while JJ was still rising at 154. Jackson would've probably taken that version of Hagler, who was too old and slow by that time. Whereas a prime Hagler would've absorbed some shots enroute to mowing Julian down Hearns-style.
Jackson, of all fighters, may have even hung a real knockdown on Marv, or at least, as has been said, rocked him, before being dispatched in 4 or less. It would have been a VERY exciting melee while it lasted.
Marvin was a strong guy who if anyone can take the Jackson punch it is Marvin. He would do what he did against Hearns and keep his chin tucked in and throw punches and win a 3 or 4 round shootout. Against big punches Marvin would have the same tactic. I don't think Julian lands as much as Hearns, but he would be dangerous the whole time.
Julian Jackson in his prime and before was not as chinny as people consider him . His loss to McCallum was a Calzaghe like stoppage helped by a Hopkinsic act (and I don't mean a butt). Jackson was better than Mugabi at every aspect so why do people think it would have only lasted 2-4 rounds ?
Mugabi hung with Hagler even for 7 rounds or so. Jackson would at least does that well, and if he doesn't break his hand like Hearns, who knows.
Hagler was past his prime for Mugabi I might add. That version of Marv might struggle with The Hawk. The younger Hagler that massacred Sibson would stop Julian in 7-8 rounds, and Jackson is one of my all time favs.
Mugabi was like a bull, Hagler knew had to wear him down, then take him. With Jackson it would be a shootout. Hagler would attack & if he can absorb the punch. Hagler would turn up the pressure, just like the Hearns fight. Jackson doesn't box as well as Hearns, plus Jackson can be countered. A big Hagler shot & Jackson is in real trouble. Hagler wont let him off the hook. Stoppage within three rounds.