Coach Norv Turner said the team won't know how long Mathews will be out until after the surgery, but he's almost certain to miss at least the first week of the regular season.
"My hopes for Ryan have not changed," Turner said. "I still expect him to have a great year."
Mathews rushed for 1,091 yards last season, but the 24-year-old missed six games in his first two seasons due to injuries before Thursday's latest setback.
The Chargers don't have a clear backup behind Mathews. Veterans Jackie Battle, Ronnie Brown and fullback Le'Ron McClain all signed as free agents in the offseason. Undrafted rookie Edwin Baker (Michigan State) and second-year back Curtis Brinkley are also intriguing yet unproven options.
-- The Sports Xchange