The heir apparent to LaDainian Tomlinson, Mathews was expected to be at Sunday's late-afternoon practice. He missed seven practices, including three for rookies and selected veterans last week.
San Diego moved up 16 spots in the draft to take Mathews with the 12th pick overall. He'll replace Tomlinson, the 2006 NFL MVP who was released in the offseason and then signed with the New York Jets.
Mathews rushed for 3,280 yards in three seasons at Fresno State, scoring a school-record 39 touchdowns and averaging 6.1 yards per carry.
Last season as a junior he rushed for 100 yards against three Top 25 teams, Cincinnati, Boise State and Wisconsin. He ran for 234 yards against Boise State, scoring on runs of 69, 60 and 68 yards.
The Chargers still have three holdouts. Linebacker Shawne Merriman, left tackle Marcus McNeill and Pro Bowl wide receiver Vincent Jackson are unhappy about their status as restricted free agents.
McNeill and Jackson are expected to stay away through the first 10 games of the season, returning for the final six games in order to accrue a season toward unrestricted free agency. It's not known when Merriman will report.