Galaxy head coach Bruce Arena said Riley would need an operation after tearing his meniscus, an injury that could sideline him for at least two months.
“A.J. will be all right,” Arena said after Tuesday morning's training session at StubHub Center. “It might take him through the week to get through his injury. James is probably going to miss some time.”
The Galaxy have a bye this week and return to action in the April 6 SuperClasico against Chivas USA.
Riley, a veteran defender who signed with LA after a preseason trial, was hurt in the 30th minute of Saturday afternoon's match after he was challenged on the right wing by RSL midfielder Ned Grabavoy.
DeLaGarza suffered an apparent contusion on his left ankle when he was raked by Álvaro Saborío in the 63rd minute, an injury that was aggravated a few minutes later in contact with Olmes García, forcing him to leave the game.
DeLaGarza is the Galaxy's first-choice right back, but at RSL he played in his old spot, partnering Omar Gonzalez in the middle. Riley, brought in to provide depth at outside back, was playing on the right after starting on the left in LA's first three matches.
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Dan Gargan, who made his Galaxy debut when he came on for Riley in the 38th minute, becomes the primary depth at outside back. He's still working on fitness after dealing with minor injuries during preseason and said he played Saturday with a minor groin strain.
“I think I've been training well, and I think it was a little but unfortunate with a couple injuries that I had in preseason,” Gargan said, “but I think Bruce has plenty of faith in me to get the job done. Obviously, you never want to see guys go down ... that's why Bruce put together the roster that he did.”