Rodney Wallace also scored in Portland's third victory at home this season. The win gave Portland (7-7-9) a share of fifth place in the Western Conference with 11 matches left.
Chivas (6-11-5), sitting in last place in the West, has lost four straight after a four-game winning streak.
Earlier this week the focus was on Portland because of the Major League Soccer All-Star game.
Timbers coach Caleb Porter led the All-Stars to a 2-1 victory Wednesday night over German power Bayern Munich. L.A. Galaxy forward Landon Donovan scored the winning goal in his record 14th All-Star Game appearance, and the next day announced he was retiring from soccer at the end of this season.
The Timbers had three players on the All-Star roster, but then it was back to business quickly as the team makes the push toward the playoffs.
Coming off a 3-1 loss at the L.A Galaxy last weekend, Portland entered the game in seventh place in the West.
Valeri fooled a pair of defenders and got off a long shot that sailed past the outstretched arms of Chivas goalkeeper Dan Kennedy. Valeri had the assist on Donovan's goal in the All-Star game.
With seven goals this season, Valeri is the second-leading scorer on the team behind Maximiliano Urruti with eight. Valeri leads the team with eight assists.
Liam Ridgewell thought he had a rebound goal for the Timbers in the 25th minute, but it was ruled offsides.
Portland upped its advantage to 2-0 with Wallace's bouncing goal from atop the box. Wallace danced on the sidelines to celebration his first goal of the season.
The Timbers were without starting midfielder Darlington Nagbe, who missed his second game with a knee injury.