Charles, who rushed for 1,120 yards last season, took a short pass from Matt Cassel and was just going out of bounds when Allen pushed him out with a hit around the ankles. He seemed to be holding his right arm but it was not immediately clear where Charles was hurt.
The Chiefs had no comment on him after the game.
Kansas City lost 20-17.
Charles set a team one-game rushing record against Denver last year with 259 yards.
Information from The Associated Press contributed to this report.