Gotta say, I'm really loving this idea that Jinbe/Franky are taking on the strongest Headliners below the calamities. It really solidifies their place strength wise in the crew all things considering.
Not only that, but a recovering Jack doesn't seem to be this major accomplishmen/feat for any character. I pretty much see Luffy fighting his hybrid as a warmup to test out the new Haki he has. Considering Luffy has defeated YCs before, it shouldn't be a problem and combined with the damage already given to Jack by the Mink kings, should make that fight go relatively quick if it does.
Plus, giving Luffy a warmup fight is an Oda staple at this point so I wouldn't be shocked if that's the case, not even including Luffys proclamations to kick Jack's ass back in Zou. Narrative payoff is King in this story so im thinking that may be the case now