I think the central defensive partnership is decided upon based our opponents. Mori struggles, it's fair to say against the more physical sides, hence why he was dropped for the last two games. Whereas Jag's legs have pretty much gone, so he hasn't been playing against the more dynamic, pressing, quicker teams.
I wouldn't be surprised to see us go back to a Williams / Mori partnership in the derby, or indeed a back three.
Oh and @Ross
it's blatantly not "Managerial dick swinging", it was plainly obvious to anyone and everyone that Jags was woefully out of form earlier this season.
I'm glad he's done well in the last two games, but I won't let sentiment cloud my judgement... he shouldn't be a starting option for us next season.