The move comes after Ben Alnwick, on loan from Spurs, suffered a gash to his face in City's 4-1 Carling Cup defeat by Sunderland on Monday.
Australian summer signing Michael Theoklitos has not played since the 7-1 opening day defeat to Colchester.
Forster, 21, has been on loan with League One team Bristol Rovers since the start of this season.