Rivers conducted a quarterback clinic in the first half while taking apart the Bills' defense and leading the Chargers to a 31-20 victory at New Era Field. He also spoiled Allen's first NFL start after he was selected seventh overall in the draft.
In his 15th season, at age 36, Rivers completed 23-of-27 passes for 256 yards and three touchdowns. He was 15-of-16 for 178 yards and three scores in the first half as the Chargers built a 28-3 lead and never looked back for their first win of the season.
For two quarters, the Bills looked like a team that many picked to be the worst in the NFL before playing better in the second half. Buffalo handed the keys to the offense to its rookie quarterback in an effort to develop him on the fly.
Allen was 18-of-33 for 235 yards and a three-yard touchdown pass to Kelvin Benjamin in the final minute. He threw two interceptions and was sacked six times.
Melvin Gordon had three touchdowns -- one rushing and two receiving -- for the Chargers. Los Angeles was the better team from the start. The Chargers scored early and often and ended any doubt about the outcome before the start of the third quarter.
The Chargers were intent on teeing off on the young quarterback until the Bills' porous offensive line proved it could protect the quarterback. The unit was justifiably vilified last week after Nathan Peterman had no time to throw against Baltimore.
Different quarterback, different defense, same result for Buffalo.
Allen dropped back to pass on his first series Sunday and was promptly sacked for a three-yard loss. He was sacked again on his fifth play after Derwin James nearly stole the handoff. LeSean McCoy had little room to roam for the second straight week before leaving with a rib injury.
Buffalo was trailing by two touchdowns by the time Allen looked comfortable. But he showed enough in the first quarter to justify the Bills' decision to get him into the lineup. He threw for 47 yards in the first quarter, nearly double Peterman's output last week.
Allen was sacked four times in the first half. The Chargers deserved credit for getting to the quarterback, but the rookie also had moments in which he held onto the ball too long. He missed open receivers and made errant throws, as Peterman did in the opener.
Meanwhile, Rivers completed his first seven passes for 90 yards and two touchdowns in helping the Chargers build a 21-3 lead. Los Angeles scored three touchdowns on their first four possessions and was primed to roll over Buffalo the way Baltimore did last week in a 47-3 win.
It wasn't all bad for the Bills.
Allen led them on their first touchdown drive of the season to bring the Bills within 28-13 in the third quarter. He completed a 57-yard pass to Zay Jones and showed off his mobility and athleticism on several running plays near the goal line before Chris Ivory found the end zone.
The Chargers scored 28 points in a loss last week to the Chiefs while rolling up more than 500 yards in total offense. The outcome against Kansas City might have been different in the opener if L.A. didn't have so many dropped passes.
The Chargers started the way they wanted Sunday. They scored on their second and third possessions. Rivers found Mike Williams with a 10-yard touchdown pass over the middle, and Gordon coasted for a 20-yard score for a 14-0 lead.
Rivers didn't throw an incomplete pass until 8:37 remained in the first half Sunday before completing another eight straight. Los Angeles controlled the line of scrimmage on both sides of the ball, giving Rivers the time he needed in the pocket while taking the same away from Allen.
Gordon's touchdown run came one play after the Chargers gave Rivers enough time to find Williams for 17 yards on third-and-12.
For the second straight week, the game turned into a preseason game for the Bills. With the outcome settled, they focused on getting Allen as much time as possible in an effort to accelerate his development. The Bills appear to be a long way from winning.
They were no match for the Chargers.