Roethlisberger completed 22 of 42 passes for 285 yards and three second-half touchdowns. But he also threw an interception and had a botched screen pass turn into an easy San Diego score as Pittsburgh (7-6) hardly played like a team readying for a postseason run.
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 focus lacking for much of this season. The injury-ravaged offensive line kept Rivers out of harm's way and 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.
It wasn't exactly the triumphant return the Steelers were expecting from their franchise quarterback. Pittsburgh hoped Roethlisberger's comeback combined with last week's emotional win at Baltimore would serve as fuel for another December push.
Instead, the previously inept Chargers pushed back.
San Diego took a 13-3 lead during an ugly first half that made it difficult to tell which team was trying to get back into the postseason and which was simply playing out the season.
Roethlisberger, less than a month removed a punishing hit against Kansas City that nearly sent one of his ribs into his aorta, struggled getting loose. His second pass stuck in his hand and ended up spiking into the ground. His third short-hopped an open Mike Wallace.
Things never got better. The running game got no traction Roethlisberger actually led the Steelers in rushing in the first half and Pittsburgh's receivers did little to help their rusty quarterback.
Wallace let a 50-yard pass that would have gotten Pittsburgh out of an early hole bounce harmlessly off his fingertips. Brown followed suit on Pittsburgh's next drive, a 30-yard bullet thudding off his chest.
It led to a familiar pattern. The Steelers got pinned deep in their territory, sputtered out in a handful of plays, giving San Diego the ball near midfield.
Eventually, the Chargers took advantage. Playing conservatively while trying to protect Rivers behind that patchwork offensive line, San Diego got going late in the half.
Rivers hit a wide-open Alexander on a 39-yard touchdown pass and Nick Novak added a 39-yard field goal to make it 13-0.
Pittsburgh briefly rallied, getting across midfield for the first time during a last-second drive. Shaun Suisham made a 49-yard field goal at the halftime gun.
Any boost for the Steelers disappeared during the first 10 minutes of the third quarter.
San Diego converted five straight third downs during a clock-chewing, momentum-sapping drive, the final conversion on a 3-yard pass from Rivers to Floyd to make it 20-3.
The 17-play march ate up nearly 10 minutes. And San Diego needed barely 10 seconds to put the game away.
Backed up yet again, Roethlisberger tried to hit Brown on a screen pass. It caromed off 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.
Chargers 34, Steelers 24
San Diego 3 10 14 734
Pittsburgh 0 3 7 1424
SDFG Novak 51, 3:06.
SDAlexander 39 pass from Rivers (Novak kick), 3:40.
SDFG Novak 39, :50.
PitFG Suisham 49, :00.
SDFloyd 3 pass from Rivers (Novak kick), 5:28.
SDJammer fumble recovery in end zone (Novak kick), 5:16.
PitWallace 40 pass from Roethlisberger (Suisham kick), 2:33.
SDAlexander 15 pass from Rivers (Novak kick), 9:46.
PitWallace 11 pass from Roethlisberger (Suisham kick), 6:07.
PitA.Brown 1 pass from Roethlisberger (Suisham kick), :58.
First downs 19 18
Total Net Yards 294 340
Rushes-yards 36-94 17-69
Passing 200 271
Punt Returns 3-35 1-0
Kickoff Returns 3-24 5-113
Interceptions Ret. 1-0 0-0
Comp-Att-Int 21-42-0 22-42-1
Sacked-Yards Lost 1-0 2-14
Punts 7-37.4 7-47.4
Fumbles-Lost 0-0 2-1
Penalties-Yards 6-51 8-52
Time of Possession 36:46 23:14
RUSHINGSan Diego, Mathews 25-65, Brown 2-17, McClain 3-4, Weddle 1-4, Rivers 3-3, Battle 2-1. Pittsburgh, Dwyer 8-32, Roethlisberger 5-31, Rainey 1-6, Redman 2-0, Wallace 1-0.
PASSINGSan Diego, Rivers 21-41-0-200, Alexander 0-1-0-0. Pittsburgh, Roethlisberger 22-42-1-285.
RECEIVINGSan Diego, Alexander 7-88, Spurlock 7-64, Gates 3-31, Floyd 3-10, Brown 1-7. Pittsburgh, Wallace 7-112, Miller 5-66, A.Brown 4-35, Dwyer 3-18, Sanders 2-36, Burress 1-18.
MISSED FIELD GOALSNone.