Charles avoided other ligament damage, a source told ESPN NFL Insider Adam Schefter, but the MRI on Monday confirmed the ACL tear.
The Chiefs are bringing former New Orleans Saints running back Pierre Thomas in for a workout, a source told Schefter. The team is also expected to work out Ben Tate. Kansas City has used Charcandrick West as its backup for Charles. West had seven rushes for 31 yards Sunday.
Charles was injured without contact while making a cut during the game against the Chicago Bears. He put no weight on the leg as he was helped from the field. Charles missed all but two games of the 2011 season because of a torn ACL in his left knee.
Coach Andy Reid said earlier Monday that "the worst case is the ACL is torn. That's probably the direction it's heading.''
The Chiefs led the Bears 17-3 when Charles exited the game. They collapsed without him and lost 18-17 after the Bears scored two touchdowns in the final 3:05.
"I talked to Jamaal yesterday, and he was obviously down, but he is positive about coming back, getting himself back,'' Reid said before learning the diagnosis. "That's where his energy will be directed -- getting through the surgery and getting back to where he can play.''
Reid sounded optimistic that Charles would make it back eventually.
"He's an amazing guy that way,'' Reid said. "He's one of those rare athletes that you have an opportunity to coach. They're genetically and mentally strong, and they can do things most people can't do when it comes to their profession.''
ESPN staff writer Adam Teicher and The Associated Press contributed to this report.