Quite possibly not, though he's one of those fighters who gets matched appropriately most of the time, not like Mayweather, Pacman, Mosley Moorer etc.
I thought he beat Ishimatsu, but as you say if he did get the decision then fought DeJesus, chances are DeJesus would have been victorious at that point in time. Prime for prime though is different IMO. I think in this match up you have a counter-puncher, in DeJesus, and a 'pure' boxer, in Buchanan. DeJesus normally took his stand in the middle of the ring looking to slip in off his opponents shots and land some nice combo's and that left hook, but against a guy like Buchanan - who would be jabbing and moving - it would be hard to spot the openings for DeJesus to slide into range with, as Buchanan's angle would have changed after his shots. Buchanan should be triumphant but it is close, purely because they are quite equally matched, and Buchanan can't move for every second of every round and DeJesus would get in some hard left hooks.
I'd lean toward DeJesus in this. Buchanan tended to get sucked into rough fights, and DeJesus was a sharper, harder puncher.
The silky-smooth DeJesus that out-classed Duran the first time would give Kenny, or any 135-pounder, BIG BIG trouble.