This may sound silly, but I would play Micheal Carrick at centre back along with Smalling, He likes to play just in front of the back four, so he could easilly fill in. Hes fit, strong, good in the air, and what you really need when playing at the back, you must be able to read the game from the back. and Carrick does this with ease, he always seems to know what going to happen early, so his reading is quicker than most. The only other one left is Johnny Evens, but hes not been very good this season, but then again, this could kick-start his season. Hes maybe worth a chance, But I'm sure, Sir Alex, will have this position filled as we speak, and as usual, with the right person.