In just their second year in the league, Atlanta dominated the Timbers at Mercedes-Benz Stadium, scoring a goal in each half.
MLS' most valuable player Josef Martinez settled Atlanta's nerves by scoring in the 39th minute, his 35th goal of the season.
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The Venezuela striker also had an assist in the second goal early in the second half, flicking on a glancing header to full back Franco Escobar, who ghosted in at the far post to score from point-blank range.
The victory provided a perfect send-off for Atlanta coach Gerardo 'Tata' Martino, who is leaving to take charge, according to multiple media reports, of the Mexico national team.
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