The winger started in the 2-0 CONCACAF Champions League group stage win over Costa Rican club Cartagines but had to be subbed off just before half-time with a slight hamstring strain.
Now he has to get his body back to 100% before he can again be considered by head coach Bruce Arena.
"I'm not sure [how bad it is] ...," he said following the Galaxy's win at the StubHub Center. "It didn't feel like an extreme pain. I just felt it tear a little bit. It's frustrating.
"It's really frustrating," he added. "And, obviously, things happen for a reason, but sometimes you ask, 'What's the lesson in this?' I don't know, just going to keep my head up, and hopefully the MRI isn't too bad and I can continue training and do fitness and stuff like that.
"I don't think I'll be out too long. I think maybe a few weeks."