As reported by Luis Bueno (link) LA Galaxy captain Landon Donovan missed practice on Thursday recovering from a Quadriceps injury. He played through the offseason with Everton, came back with a nasty bug, then played four games straight. It's possible all the wear caught up with him during the time off.
"Since he’s come back, he’s had a bunch and we’ve kept him out of training," Arena told reporters on Thursday.
There's a high possibility this will come to nothing, and Donovan will be on the pitch on Saturday. He did practice last Saturday at a Season Ticket Holder event, so it's not an injury directly caused by the last match. The team will know more on Friday, and at this point in the season it's better to be proactive and take precaution. Landon Donovan and David Beckham both played through injuries during MLS Cup, but the more of these slight discomforts that can be relieved the better; lest they creep up again latter in the season.
After Satuday's match, the Galaxy next have to travel for the first time in MLS play this season; going to Kansas City. Certainly that could be a factor too, better to rest this week than try to recover on the road.