I think Jiraiya's outclassed physically and mentally. Itachi isn't as fast as Minato, but his shunshin was still ridiculously fast compared to other ninja, as it kept shocking Bee. Combined with genjutsu, which can control Gambabuta and others, and Jiraiya is going to go down well before he can attain Sage Mode, maybe to Amaterasu.
In order to win this match, Jiraiya needs to start in Sage Mode and have a decent amount of distance between Itachi and himself. Then he can use his strength of guerilla warfare and play to Itachi's lack of chakra-efficient long range jutsu, as the only ones that we really know about are Amaterasu and the Magatamas.