The Sheffield-born goalkeeper is set to go straight into the side for Lucas, and the club do not know whether his injury is a serious one at this point.
Weaver, understudy to David James and Kasper Schmeichel has found opportunities at The City of Manchester Stadium limited in recent years and will stay at Hillsborough until December 17.
City do have a clause that he can go back across the Pennines if Stuart Pearce needs him at short notice.