San Diego Chargers free agent running back Ryan Mathews is an interesting case. On one hand, he's rushed for at least 1,000 yards and six touchdowns in the two seasons he remained mostly healthy. But on the other, he's missed a total of 20 games in his five NFL seasons. Determining his value on the open market is incredibly difficult.

Despite the injury risks, though, the Chargers are reportedly interested in retaining Mathews this off-season. But they won't re-sign him without some built-in protection.

"San Diego has discussed an incentive-laden deal with the running back, the parameters taking into account his equally undeniable on-field ability and injury history," Michael Gehlken of the San Diego Union-Tribune wrote. "The coming hours won't lack interest, as free agency begins Tuesday at 1 p.m. PT.

"Answers are coming.

"It is possible Mathews signs a 1-year, prove-it deal with the Chargers; his desire to remain in San Diego is well known. No such contract, though, had been finalized as of Monday night, a source said, so the calendar will turn another day toward resolution."

The value of running backs has declined over recent seasons, meaning that Mathews will not be too pricey to keep around. Mark Ingram, who has also had his share of injuries, recently signed a four-year deal with the New Orleans Saints for $16 million. San Diego could use that pay scale as a relative parallel for Mathews' contract.