Rams boss George Burley moved for Caldwell as a solution to his squad’s current defensive injury crisis, which sees Dave Walton and Youl Mawene on the sidelines.
The move is the fourth time the younger Caldwell brother has left St James’ Park on loan, as he has previously spent a month at Darlington, four months at Hibs and the whole of last season at Coventry City.
Although a full Scotland international he is yet to make a first team appearance for us, and after being left out of the squad for the Asia Cup last month he told a Scottish newspaper that he was keen to go out on loan again as he thought he would have very limited opportunities with United.