The Sounders have a big task against the rival Timbers, with Clint Dempsey and company looking to overturn a goal deficit on the road.
The Sounders pulled a goal back late to cut the deficit to 2-1, but the loss at home does not bode well for Sigi Schmid's side.
Going into the match in Seattle, the Timbers had never held a lead over the Sounders at CenturyLink Field, and the club's first win at the home of its biggest rival will certainly be a boost heading into the second leg.
The Timbers will also be boosted by the fact that Lamar Neagle's yellow card in the first leg rules the striker out of action for the return fixture in Portland.
Seattle will be worried over the fitness of right back DeAndre Yedlin (ankle) and striker Obafimi Martins (groin) who both missed out on the first leg through injury, although both have been able to do limited training early in the week. Defenders Zack Scott (cracked rib) and Leo Gonzalez (hamstring) are also both concerns for the Sounders as they head to Portland.
Schmid will likely draw some confidence from his team's attacking intent in the first leg, with the Sounders getting off 20 shots and winning 11 corners in the opening 90 minutes of the tie, only to come up short until the 90th minute when Osvaldo Alonso was able to find the net.
Portland, however, has not been a place where goals or wins have come easily for visiting teams. The Timbers currently hold a home streak without conceding that stretches all the way back to Aug. 21, and Portland has not lost at home since March.
The home shutout streak includes a 1-0 win over the Sounders on Oct. 13.