San Diego ? It’s a good thing the Bus is back, because now the Pittsburgh Steelers could be without Ben Roethlisberger.

Roethlisberger was helped off the field because of an apparent knee injury shortly before Jeff Reed kicked a 40-yard field goal with six seconds left to give the Steelers a 24-22 victory Monday over the San Diego Chargers.

Roethlisberger thrust both fists in the air in celebration from the bench, his left leg wrapped in ice and bandages.

“He laid it on the line,” receiver Hines Ward said of the second-year quarterback. “He’s lying there hurt on the ground, and he said to me, ‘At least we’re in field-goal range.”‘

Steelers coach Bill Cowher had no information on the exact nature or severity of his QB’s injury.

“It looked bad from what I saw up on the screen,” Cowher said.

Trailing 22-21 in the fourth quarter, Roethlisberger moved the Steelers into San Diego territory and had completed a nine-yard pass to Antwaan Randle El when he was hit on the knee by the helmet of Chargers rookie lineman Luis Castillo.

Roethlisberger, whose knee was bent back awkwardly on the hit, was driven off the field on a cart after the final gun.

Pittsburgh's James Harrison (92) hurdles San Diego's LaDanian Tomlinson during an interception return. The Steelers beat the Chargers, 24-22, Monday in San Diego.

Roethlisberger was replaced by Charlie Batch, who has played only a handful of downs the last three years. Batch handed off three straight times to Jerome Bettis before Reed’s kick.

The Bus carried seven times for 21 yards on the winning drive.

“I went to Charlie and said, ‘Give it to me, look it all the way in, follow me as I go through, and I’ll do the rest,”‘ said Bettis, who missed the first three games because of a calf injury sustained during the exhibition season.

Roethlisberger ran for one touchdown and threw for another, and Bettis rushed for a TD in his season debut for the Steelers (3-1). San Diego (2-3) had its two-game winning streak snapped.

“I feel great, I feel fresh,” Bettis said.

San Diego had gone ahead on LaDainian Tomlinson’s two-yard run with 4:42 left. Tomlinson was stuffed on a two-point conversion attempt that would have given the Chargers a 24-21 lead.

“It’s 50-50 there,” Chargers quarterback Drew Brees said. “Give it to the best running back in the league or throw it to the best tight end in the league,” he said, referring to Antonio Gates.

Tomlinson extended his NFL record by scoring a rushing touchdown in 17 straight games.

Stung by a loss to New England 15 days earlier and coming off their bye, the Steelers bounced back with a crisp game by Roethlisberger and the defense, which made Tomlinson and the rest of the San Diego offense look average.

Tomlinson was held to 62 yards on 18 carries. Brees threw his first interception in three games, which set up Bettis’ TD run.

“That was probably the best defense I’ve faced as far as the run game in my career,” Tomlinson said.