Boxing is part of his arsenal and I believe he was a pro-boxer (short record of 1-1) but his strength lies in MMA cos he's a BJJ black belt too (if you notice, he's sincere to all of his opponents. Taps their hands to see if they're ready. Stops to see if they're okay etc).
Pure boxers or pure BJJ guys/wrestlers normally do rubbish in MMA if they know nothing else. Even Marcus Davis, one time pro-boxer with a record of 17-1-2 had to learn other things before entering the octagon.
Hagler is one of my favs from the era but he didn't have to worry about shoots, kicks, flying armbars etc.
MMA and boxing are both great sports. So is Judo, Greco Wrestling, Fresstyle Wrestling, BJJ - man I love em all.