The league's disciplinary committee said the one-game suspension resulted from Arena leaving the coaching area when he was arguing a call near the end of the Galaxy's 1-0 loss to the Timbers last Sunday in Portland.
It appeared the Galaxy had tied the game in stoppage time on a header goal by Robbie Keane, but officials ruled that Keane was offside, sparking a vehement argument from Keane and Arena.
After the Galaxy practiced Friday, Arena said "the best thing is to say no comment at this point" and "there will be a time and place for me to discuss that with the right people."
Dave Sarachan, the Galaxy's associate head coach, is expected to guide the club Sunday when the Galaxy and Chivas USA meet for the third and final time this season in what's known as the "Super Clasico" series.
The two teams share StubHub Center in Carson as their home venue. The Galaxy is the home team for the game Sunday at 2 p.m.
Sarachan said Arena's suspension "doesn't change" the Galaxy's outlook. "We've had a great week of training and now it's up to the guys. They're well-prepared, we've assessed what we need to do against Chivas and now we'll perform on Sunday."
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