Questionable for fans, maybe, but the FO and coaches must have their reasons.
This news increases my concern for the health of David Thomas. He may be headed for IR. While a guard wouldn't replace his function, it could be that we're looking at other ways of beefing up the run blocking package for the stretch and the playoff run. It could be that this guy was someone they thought could help in those rotations.
A Guard lined up at Tackle with a Tackle eligible on the outside to protect him could help the run game. The line would be OT-OG-OC-OG-OG-OT with Streif being the tackle eligible or it could flip flop with Bushrod being the Tackle eligible.