Charles turned his right ankle early in practice on Monday.
"His ankle ligaments were fine," Chiefs trainer Rick Burkholder said. "He had some pain on the outside bone of his foot. We did an X-ray. That was negative. There's a tendon that attaches right at that spot and that's what is sore right now. That's why we call it a strain."
Charles walked down the long, grassy hill to the practice fields Tuesday without a boot or a noticeable limp. He spent some time with the training staff off to the side, riding a stationary bike but only using the arm levers, before leaving the field for the rest of practice.
Burkholder said Charles felt some soreness up his leg but that it was expected.
"People are speculating Lisfranc and all that," Burkholder said of the mid-foot injury that caused problems for former Chiefs running back Larry Johnson. "He doesn't have any discomfort there. I've had him see two orthopedic surgeons and everyone agrees he has a mild foot strain."
It's unclear how long Charles will be out of practice, but it appears unlikely that he will play in Friday night's preseason game against San Francisco at Arrowhead Stadium.
The Chiefs leaned heavily on Charles in their preseason opener last week. He touched the ball on eight of the 14 plays that the No. 1 offense ran on its opening touchdown drive.
"No difference. Next guy has to step up," Chiefs offensive coordinator Doug Pederson said. "They're getting a lot of reps right now, so we just keep going. Nothing changes."
Rookie running back Knile Davis took most of the carries with the first-team offense on Tuesday. Shaun Draughn and Cyrus Gray continued their battle for the No. 3 job. Chiefs quarterback Alex Smith said that good teams are able to overcome an injury to one of their most productive players, and that somebody else will have to step up.
"He's a special player. Kind of stating the obvious," Smith said. "Jamaal is a special player but we have to find a way. ... But no question, he'll be missed."