Mathews was injured on his first carry of the preseason in the first quarter of Thursday night's game against the Green Bay Packers.
Coach Norv Turner has said he'll be out four to six weeks.
"It's just unfortunate," quarterback Philip Rivers told the San Diego Union-Tribune. "His first carry of the season. He'll stay strong. We'll keep him strong and upbeat because I know how excited he was about the season. He was openly commenting about how excited he was and how hard he's trained. He'll still bounce back and have a great year."
Mathews was involved in a car crash on Monday, but said he was only banged-up and was able to play Thursday.
Mathews, who replaced LaDainian Tomlinson as San Diego's feature back in 2010, ran for 1,091 yards and six touchdowns last season. He added 50 catches for 455 yards.
Since being drafted 12th overall in 2010, Mathews has had trouble staying healthy. He missed four games in his rookie campaign and two more last season.
"That's why Ronnie Brown's here," Turner told the newspaper. "I think you're going to have situations where you're not going to have everybody. That's what I've been talking about since April after the draft and after we signed these players, we've protected ourselves in those situations.
"If someone goes down, we have answers. We have guys who can step in and go."