I actually don't think this is an issue at all. I honestly wouldn't even be surprised if there was a late change this week and Billy switched out of the six to make way for Gamble (or hopefully less likely, Mam). [Author's Note: Late Changes =/= Conspiracy]
As much as I'd prefer Gamble, I also can't really argue with Billy's selection. He won the 6 jumper against Souths on preseason form, and was as good in that game as Kelly, and might have been considered unlucky to cop the demotion to the bench for Dogs game. He hasn't done much wrong since then, even if he hasn't set the world on fire, and from all reports Gamble isn't exactly killing it in Q Cup, so he is a fairly logical first replacement for Kelly.
I think Walters Sr is aware that Walters Jr's value lies in that utility role, and as soon as a better option is available or presents itself for the five-eighth spot, Bill will ride the pine again (or shift to starting hooker, should Turpin finally, deservedly, get the hook).