SANTA CLARA, Calif. -- After landing on the injury report Thursday afternoon with a calf issue, San Francisco 49ers running back Christian McCaffrey was back on the field for Friday's practice but did not actually participate in the session.

McCaffrey is officially listed as questionable for Sunday's regular season opener against the Seattle Seahawks . Coach Kyle Shanahan declined to offer specifics of McCaffrey's injury on Friday but did say the calf issue came up during Thursday's practice .

On Friday, McCaffrey was in uniform for the early portion of practice open to media. He did his usual stretching and warmup routine on the side under the watch of Mike Sola, the team's director of reconditioning. While it's not unusual for McCaffrey to warm up on the side, he does usually j

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