Ferguson has so far rotated Anders Lindegaard and David De Gea this term.
But Boz, who had two spells at United, told talkSPORT: "The goalkeeping situation is one they have to sort out very soon.
"We saw it least season, he was swapping Lindegaard and De Gea up until Christmas. This season he is doing the same.
"From a defensive stability point of view, it is imperative you keep playing the same back four and goalkeeper as much as you possibly can. That continuity, that understanding that you develop as a goalkeeper with your defenders, is priceless.
"The sooner he makes the decision, whether it be Lindegaard or De Gea, the better it will be for that defensive unit."