Mathews was scheduled to undergo an MRI, a source told ESPN NFL Insider Adam Schefter. The Chargers were concerned Mathews could have a sprained MCL, the source said.
With 12:22 left in the game, Mathews suffered the injury while trying to recover a fumble. Eddie Royal recovered the ball.
Mathews was attended to by trainers before being taken off of the field. The Fresno State product is in the final year of his contract, and was looking to put up big numbers in this season in order to earn a more lucrative new deal.
Mathews, 26, finished the game 31 yards on 11 carries.
Mathews played a full, 16-game season for the first time in his NFL career in 2013, finishing with a career-best 1,255 yards. However, a lingering ankle injury limited his effectiveness in the playoffs, and he sat out most of San Diego's AFC divisional playoff loss at Denver.