Lee Thomas is expected to miss the first three months of the Top 14 season in France after having knee surgery.
The 27-year-old Welshman helped Lyon to promotion to France's top league by winning the second-tier Pro D2 last season - his first at the club.
Thomas moved to Lyon in August 2010 after spending four seasons with Sale Sharks in the English Premiership.
"We hope to welcome him back for the home game with Montpelier in November," said a Lyon spokesman.
Before joining Sale in 2006, Thomas played for Cardiff Blues, Celtic Warriors, Pontypridd and Cardiff. He also captained Wales at all age levels up to Under-21, but has yet to win a full cap.