"We wanted to come out and prove a point, healthy or not," goalkeeper Nick Rimando said.
The Galaxy knocked RSL out of the playoffs here last season, and were poised for a fourth straight home win against its short-handed rival after Edson Buddle the man who has scored 10 goals in 13 career games against RSL knocked home a header in the 71st minute.
But the Galaxy's Sean Franklin committed a ghastly own-goal just two minutes later, before RSL's Javier Morales and Fabian Espindola piled on two more goals in the 80th and 85th minutes, respectively, to snap the Galaxy's 18-game regular-season home unbeaten streak.
"Wish we'd have done it a couple of months ago," RSL's Kyle Beckerman said. "It would've been better. But it was a nice consolation, I guess."
It was the third straight season-opening victory for RSL, which hopes to overhaul the Galaxy this season for the Major League Soccer championship. And it was the kind of game the team might view as a landmark months from now, considering the way it rallied with a lineup lacking some of its usual punch.
Both Morales and forward Alvaro Saborio did not enter the game until the 67th minute, their playing time limited as they continue to recover from offseason injuries. Two other regular starters midfielder Will Johnson and defender Nat Borchers did not play at all, for the same reason.
Meanwhile, the Galaxy came out practically full-strength after playing to a dramatic draw in a CONCACAF Champions League quarterfinal game in Toronto on Wednesday night. That meant all the stars were out Landon Donovan, David Beckham, Robbie Keane and Buddle though coach Bruce Arena said that was the reason the Galaxy ultimately lost.
"This game is on me," he said. "It's my fault. I should have used fresher bodies."
Although RSL controlled most of the game with its passing and possession, the Galaxy created more dangerous chances with their ruthless counterattacks and set pieces.
Twice they hit the crossbar, forcing Rimando to save a dramatic bicycle kick off the rebound of one of them, and Keane alone failed to finish at least three sumptuous opportunities. The Galaxy tested Rimando at the end, too, desperate to come back and live up to their reputation.
But RSL hung on, launching what it hopes will be a championship season with a victory that will reverberate throughout the league.
"Really, really pleased," coach Jason Kreis said. "I feel like the guys got what we deserved."