Valeri scored twice in the second half after Rodney Wallace opened the scoring before the break as Portland accounted for San Jose Earthquakes 3-0 at Providence Park.
After coming from behind to win in San Jose on Saturday, the Timbers got the better of the Earthquakes for the second time in less than a week.
Portland now occupy the fifth and final post-season spot in the West, two points ahead of Vancouver Whitecaps with two games remaining, though the Timbers have played an extra match.
San Jose, meanwhile, sit second-bottom in the Western Conference.
Costa Rican attacker Wallace was the difference last week, scoring twice in the space of three minutes in the second half as Portland prevailed against the Earthquakes.
And he was at it again on Wednesday, breaking the deadlock in the 41st minute with his fifth goal of the season.
Wallace was in the thick of the action following the interval, winning a penalty for the Timbers after he was pulled down by San Jose defender Jason Hernandez.
Valeri made no mistake from the spot, converting the penalty to the delight of the Portland faithful.
The Argentine midfielder netted his second of the match 21 minutes later, when he tucked away Wallace's initial effort which Earthquakes goalkeeper John Busch failed to deal with.
Meanwhile, Houston Dynamo kept their season alive as Toronto's play-off hopes were dealt a huge blow.
The 10 men of Dynamo were 1-0 winners against Toronto, with Giles Barnes' 35th-minute strike the difference at BMO Field.
Kofi Sarkodie picked out Barnes with a well-weighted long ball and the 26-year-old Englishman showed tremendous strength to outmuscle his opponent before firing through the legs of Joseph Bendik from an acute angle.
Toronto were given a lifeline in the 54th minute, when A.J. Cochran was sent off for fouling Luke Moore inside the penalty area but Jermain Defoe failed to convert the resultant spot-kick.
Houston remain in the post-season hunt, sitting eighth in the Eastern Conference, four points adrift of Columbus Crew, who occupy the fifth and final play-off spot with three matches remaining.
Dominic Kinnear's men are just a point adrift of a Toronto side that have suffered back-to-back defeats and are three points behind the Crew.
