Footwork, head movement, and overall timing along with stamina is what would be the problem. He's always been sharp on the mitts (the zip with the thud) and still sharp offensively when there's nothing coming back, ala Matthysse. Throw in a Jeff Horn, though, and you see he no longer has the ability to motor on through like years past. Having said that, I give Broner quite a low percentile chance of winning this fight, that's for sure. And at Pac's age, it will be a notable good win to scoop up on paper.
I believe all of it is still there—it's the stamina and workrate due to age that concerns me. With that said, Manny needs to put some really good work on Broner early and hopefully get him out of there or takes the wind out of Broner's sails leaving him w/ little left for the championship rounds. Broner tends to come on late and has VERY GOOD success when he lets his hands go. He was busting Mikey and Jessie up in the late rounds and looked fresher than both men. In 15 round fights, Broner may have stopped Mikey and Jessie. Both guys will be EXTREMELY MOTIVATED for this fight... Manny is looking at a mega PPV showdown w/ Mayweather (and Spence, as rumored). Broner is looking at erasing all of the criticism (Manny is still an elite scalp) and setting up big pay days.