I think it is fair to say Ernie has earmarked Boxall as the go-to guy if a defender has to come off. This to date (this season) usually meant a fullback rather than a CB that Boxall apparently is. Hopefully this is enough time for Boxall in the role you are discussing as needing someone to be bedded in.
While I do not disagree with the need to eventually have a younger replacement for either Siggie or/and Dura, please bear in mind they played full games this season while the fullbacks were often subbed (stats, anyone?). This may not mean that fullbacks needed change due to physical reasons (OK, Fenton) but - conversely - neither of our CBs needed changing for physical reasons. So the argument about "ageing CBs" surely needs to be taken with a grain of salt.