After seeing what Juarez was able to do to John in the 2nd half of their fight I'd have to favor Gamboa. I think John is a really sound fighter, but Gamboa's physical abilities would be too much for John.
I also think we're only going to see Yuri get better as a pro. His amateur record speaks for itself, so the issue really isn't that he doesn't know how to move and defend. He's been lured by the weak opposition into going for broke, because he knows he can blast these guys, and he obviously doesn't fear their power either. Now of course, if he brings that same attitude into fights with top 5 fighters, he loses. I just think it's more an issue of teaching him to adjust mentally than teaching him skills he doesn't already possess. John is tricky because of his length and style. I'd pick Yuri over Juarez in an instant.