Roethlisberger was hit on the left knee by Chargers defensive end Luis Castillo after planting his front foot and throwing a 9-yard pass to Antwaan Randle El during the game-winning drive.
Charlie Batch replaced Roethlisberger -- who was later walking on the sideline -- and handed off three times to Jerome Bettis before Jeff Reed entered to kick a 40-yard field goal with six seconds remaining.
While Reed kicked the game-winner, Bettis was the game-saver. Inactive since injuring his calf in the preseason, Bettis carried 17 times for 54 yards and a 1-yard touchdown run in the first half.
Bettis's first-half touchdown gave the Steelers a 14-0 lead, but the Chargers rallied to take a 16-14 lead with the help of a controversial call by referee Jeff Triplette's crew.
The Steelers' Chidi Iwuoma recovered an apparent muff by Chargers punt returner Darren Sproles, but Triplette awarded the ball back to the Chargers, citing a rule in which a fair catch allows a return man to recover a bobble while it's still in the air.
The Chargers eventually kicked a field goal, but field goals would not beat the Steelers last night.
The vistors capitalized on the Chargers' failure to put the game away with a 16-yard touchdown pass to Heath Miller early in the fourth quarter.