Though a source familiar with the injury told ESPN NFL Insider Adam Schefter that Charles is "fine," another source told ESPN's Ed Werder that Charles will be re-evaluated Tuesday morning to determine when he'll be able to return to practice.
Charles attended Tuesday's practice but joined the rest of the injured Chiefs away from the practice field. He wasn't wearing a boot and didn't appear to be limping.
Charles was participating in an 11-on-11 portion of practice pitting the No. 1 offense against the top defense Monday when he walked off the field and spent a moment speaking with trainers. He climbed into a cart and was driven up a long hillside to the locker room.
"We'll just see how he does. Precautionary measures," Reid said. "We'll see how he does here in the next little bit -- see where he's at as far as pain or swelling. We'll see how he does."
Reid wouldn't say for sure whether Charles will play Friday night against San Francisco.
"If he's ready to go," Reid said, "he'll play."
Charles missed nearly an entire season two years ago with a torn ACL in his left knee. He hasn't been injury prone since joining the Chiefs, but not even Charles was sure whether he'd be the same player once he made it back from the left knee injury. He wound up running for at least 100 yards seven times in 2012, and more than 200 yards twice.
He's coming off a 1,500-yard season for an offense that was among the worst in the NFL, and will be counted upon heavily in Reid's system in both the running and passing game.
On the Chiefs' opening drive last Friday night in New Orleans, Charles had five carries and three catches, touching the ball on more than half of their 14 plays. Charles wound up capping the drive with a 1-yard plunge, the only TD the Chiefs scored in a 17-13 defeat.
Knile Davis, the Chiefs' third-round draft pick, stepped into Charles' place with the first-team offense for the remainder of Monday's practice. Davis had already moved past veteran Shaun Draughn and second-year running back Cyrus Gray to No. 2 on the depth chart.
"He went down. I had to get in, step in and play my role," Davis said. "[Reid's] whole goal was to build a team where if one man went down the next man would step up."
Reid said that the reps that Davis got with the first-team offense were invaluable.
The former Arkansas star was considered a first-round talent coming out of college, but he slipped down draft boards because of injury concerns and a propensity for fumbling. Already, he's shown game-breaking speed and uncanny elusiveness early in training camp.
"It was good work for Knile today, if you want to take a positive from it," Reid said. "It gives another guy an opportunity to practice. That's how I look at it. It allowed Knile, our young running back, to get good work with the ones."
Information from The Associated Press was used in this report.