Alex Evans has joined Oxford United on non-contract terms, subject to international clearance.
The 19-year-old was in the U’s squad for all but one of their pre-season games and was included on their US tour.
Evans, who was released by Cardiff City in May, will provide back-up in central defence, where Michael Duberry, Andy Whing and Harry Worley are all out.
Worley has now returned to full training, but is not ready to play.
U’s boss Chris Wilder said: “Hopefully, Alex can make us stronger going into the first two or three weeks of the season. Whether we extend (his deal) or not remains to be seen, but his attitude is great.”