The 24-year-old needs reconstructive surgery that will keep him out for at least the rest of the season and put doubts about his playing future.
Jones, who has had nearly a year out after knee ligament reconstruction on his other leg, suffered the injury in his second comeback game.
Jones has won 16 caps for Wales, the last one coming in the 2003 World Cup in Australia.
Meanwhile, the Scarlets have asked Pontypridd RFC and ex-Wales scrum-half Paul John if he is interested in a short-term role with them.
John retired from regional rugby at the end of last season, but the Scarlets have a No 9 crisis with Rhodri Jones and Dale Burn injured, and Mike Phillips and Dwayne Peel with the Wales squad.