Associated Press

KANSAS CITY—Pro Bowl running back Jamaal Charles left midway through the Kansas City Chiefs' practice Monday with what coach Andy Reid called a strained foot. X-rays were negative.

Charles was participating in an 11-on-11 portion of practice pitting the No. 1 offense against the top defense 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.