Both players suffered the injuries in the 3-0 defeat to Oxford Utd last time out, and will play no part in the next four-six weeks.
Riley has injured a quad muscle and Jennings has a tear in his calf, with Riley the most likely to return back to full fitness ahead of the other.
It is a big blow for the Stags, who face Fleetwood town this coming weekend, and who are desperate to get players off the books in order to bring in a new striker.