IMO this manga is starting to jump the shark.

[Yukimura]

While I'm not a fan of Konohamaru's debut fight being against a Pein body I think it makes sense that he could learn the Rasengan. Naruto was able to do it relatively quickly based on some explanations and demonstrations from Jiraiya. It wouldn't have taken that long for Naruto to pass that same information along to Konohamaru and maybe show him a few times, after which Konohamaru could have practiced at his leisure for all the years of the time skip. As someone pointed out already, Ebisu was Konohamaru's teacher since he was a kid and I doubt he would have let him slack on fundamentals like chakra control.

What DOES bother me about the outcome of the fight is that Konohamaru managed to hit Pain in the first place. One would assume Pain's bodies are all skilled enough to go toe to toe with jounin and should thus be very fast and quick reacting. I don't know if I like the implication that a genin has enough speed or stealth to get that close to a Pein level entity without being detected. I could buy the underestimating the kid angle if it was just Pain and Konohamaru out in the middle of nowhere but in the middle of a hostile village you'd think Pain would be more on guard against suprise attacks from hidden enemies.