The San Diego Chargers say running back Ryan Mathews has a sprained medial collateral ligament in his right knee.
Coach Mike McCoy won't say how much time Mathews is expected to miss, other than "he'll be out for a little bit of time."
McCoy says every player heals differently and "I don't want to put a timetable on anything."
Mathews was taken off the field on a cart Sunday after hurting his knee trying to recover his own fumble in the fourth quarter of a 30-21 victory against the Seattle Seahawks.
Mathews is in the final year of his contract. Last year was the first time he played all 16 regular-season games. He gained a career-high 1,255 yards.
The Chargers (1-1) have Danny Woodhead, Donald Brown and rookie Branden Oliver at running back.
Online: http://pro32.ap.org/poll and http://twitter.com/AP_NFL