Rhythm disrupted by a relentless San Diego pass rush, Roethlisberger struggled early and the Steelers never recovered in a listless -- not to mention perplexing -- 34-24 loss on Sunday.
"We got whupped," Pittsburgh linebacker Larry Foote said.
A thumping that continued a maddening trend for the Steelers (7-6), who have won on the road at Baltimore, Cincinnati and the New York Giants this year but also have fallen to Tennessee, Oakland, Cleveland and now the Chargers (5-8).
Roethlisberger completed 22-of-42 passes for 285 yards and three touchdowns in his first game back after spraining his right shoulder and dislocating a rib in an overtime win against Kansas City nearly a month ago. Most of the yards and all of the scores came after the Steelers had fallen behind by 24 points to a team that had never won in Pittsburgh during the regular season.
"I have no clue (why we're inconsistent)," Roethlisberger said. "If I knew I don't think we'd do it anymore. I thought we'd play better."
The Steelers thought they'd revived their season last Sunday in an emotional victory over the Ravens behind third-string quarterback Charlie Batch. Instead, they looked disinterested at times and sloppy at others against a team that hardly looked like it was playing out the string.
Philip Rivers threw three touchdown passes, two to Danario Alexander, and underachieving San Diego beat the Steelers on the road for the first time since the AFL and NFL merged more than 40 years ago.
"This isn't necessarily the team we thought we'd have on the field in December, but this is the type of performance we thought we'd put together," said Rivers, who completed 21-of-41 passes for 200 yards.
The Chargers have been one of the NFL's biggest disappointments after a 3-1 start evaporated into a 1-7 slide that fueled speculation coach Norv Turner and general manager A.J. Smith could be fired at season's end.
Maybe, but for an afternoon, anyway, the Chargers played with a fearlessness they've lacked during their swoon.
The injury-ravaged offensive line kept Rivers out of harm's way. The Steelers only sacked Rivers once, and all that extra time in the pocket helped him convert 12-of-22 third downs, allowing San Diego to chew up the clock and keep Roethlisberger from getting going while the game was still competitive.
"We all know that this team has played this way through large parts of many games," Turner said. "We did not make the big mistake in the football game."
Instead, it was the Steelers who couldn't seem to get out of their own way.
Mike Wallace caught two second-half touchdowns but also dropped a 50-yard rocket from Roethlisberger in the first half that would have gotten Pittsburgh out of an early hole. Antonio Brown also scored in the final seconds but saw a 40-yard pass clang off his chest in the second quarter that could have made a difference.
The running game the Steelers hoped would take pressure off Roethlisberger's right arm never materialized. Jonathan Dwyer led the Steelers with 32 yards, or one more than Roethlisberger had during five hold-your-breath scrambles.
San Diego controlled an ugly first half, taking a 13-3 lead at the break behind a 39-yard touchdown pass from Rivers to Alexander and a pair of Nick Novak field goals.
The Steelers didn't even cross midfield until a last-gasp drive to end the half ended with a 49-yard Shaun Suisham field goal.
Any boost the kick provided Pittsburgh disappeared during the first 10 minutes of the third quarter.
San Diego converted five straight third downs during a clock-chewing drive, including a 17-yard burst up the middle by Ronnie Brown on third-and-13 from the Pittsburgh 29.
The play wasn't designed to get a first down, but a pair of missed tackles helped the Chargers extend the drive. Three plays later Rivers found Michael Floyd for a 3-yard touchdown to make it 20-3.
The 17-play march ate up nearly 10 minutes. The Chargers needed barely 10 seconds to put the game away.
Backed up yet again after Floyd's score, Roethlisberger tried to hit Brown on a screen pass. The ball smacked into tight end David Paulson's rear and rolled into the end zone, where Quentin Jammer fell on it.
The play appeared to be overturned on review. Pittsburgh's offense ran back onto the field when referee Scott Green popped out from under the hood.
Instead, Green pulled a switcheroo, confirming the score to give the Chargers a 27-3 lead and a brief moment of joy in a year full of chaos.
"Yeah we've been struggling the last five weeks," tight end Antonio Gates said. "But at the end of the day it only came down to one or two plays in those games we lost. Today, we just were playing the way we were capable of playing, the way we know how to play."
Meanwhile the Steelers will have to find a way to move forward after one of the most demoralizing defeats of Tomlin's six-year tenure. Pittsburgh still controls its playoff hopes after Cincinnati lost to Dallas.
If the Steelers can win their final three games, they'll play into January. That's too far down the road to think about. At the moment, they'd just like to get back to playing something resembling competitive football.
"Those of us that have been around here, we've got to find a way to pick it up and finish strong," defensive end Brett Keisel said. "We've got to bounce back. We've got to respond from this performance."