The first one that comes to mind for me is Mauricio Herrera. The guy had a number of close fights with opponents that became top level later on in their career. Also with more careful match making he could have definitely benefitted from fighting certain opponents. The guy wasn't really top level or much of a draw, so of course his management didn't truly care about him much, but he could have probably had some of those close fights go his way and probably could have won a world title at some point.
Definitely agree with Herrera. He had a few robberies in his career too that didn't help. A few that come to mind that also could've benefitted from better management: Rigondeaux Mikey Garcia GGG from before Gamboa (?) Cotto
I think they could have benefited, but not too much in the WIN/LOSS column. I never feel like any of those guys got the short end of the stick in a decision. In the case of GGG, Rigo, Gamboa, maybe their management should have tried to get them more acclimated to USA language/culture in order for them to sell more. GGG is doing ok now, but I think a lot of it is his punching power.
That one hurts the most. World class talent with speed, power, defense, and ring IQ. He honestly could be top 5 p4p if he fought more.