Well couldn't Jones have been reserve right back?
It all depends on where Hodgson saw Jones playing. If you think back to the original squad annoucement, Hodgson confirmed he picked Jones over Ferdinand as the 4th CB. Therefore when Jagielka replaced Barry, a replacement for Cahill had already been made so when Cahill pulls out, Hodgson's decision is whether to bring in additional cover for Johnson (Jones obviously provided that even though he was picked as a cb) or another midfielder. An additional cb seemed too much.