Referee Kevin Stott waved play to continue because the ball was actually sent toward the RSL goal by midfielder Sunny. Nikolic was alone with RSL keeper Nick Rimando and easily slotted home.
Coach Jeff Cassar argued to fourth official Nima Saghafi that Nikolic was offside, not realizing what happened.
"At the time it happened, I didn't know that Sunny had nicked the ball, I thought they had sent it through," Cassar said.
Center backs David Horst and Justin Schmidt were caught flat-footed and Rimando had no chance.
"The lineman put his flag up and the ref put his whistle to his mouth and did the play on thing," Horst said.