Coach Kevin O'Connell said Wednesday that Mason has a small fracture in his thumb that underwent surgery, and the running back will be placed on injured reserve, Alec Lewis of The Athletic reports.
Mason emerged from the Vikings' Week 1 win against the Packers with a sore thumb, but the severity of the issue didn't become clear until O'Connell spoke to the media Wednesday. With Mason out for at least the next four games, Aaron Jones is set to lead the Vikings backfield for the foreseeable future, with Demond Claiborne, DeeJay Dallas and practice-squad member Jerome Ford all candidates for reps that don't go to Jones.