The Argentine playmaker scored his fourth goal of the season to keep Portland's fading playoff hopes alive.
Maximiliano Urruti's 72nd-minute near-post finish had brought the game back to level terms after Deshorn Brown scrambled the opener over the line early in the first half, setting up Valeri for a long-range stunner in the 77th that proved to be the difference in the match.
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"Valeri was special tonight," said Timbers head coach Caleb Porter. "For some reason today, and I think it's more his ability to feel a game and smell where the spaces are, he floated a lot to that left. So we felt it was the right move to shift him over there, and in the end it worked out. He was able to come inside on that right and smack a ball, score the game-winner.
The "special" adjective was echoed by Porter's on-field representative.
"It's a special goal," agreed captain Will Johnson. "We put in a lot of good work defensively - team defense - to keep them off the board after that, but you need a special play to win the game. There's no more special play than that, no more special player. He's our man.
"I fed him a good ball, and it's one of those where he's about to wind up and you're like 'no no! Pass pass pass!' and it goes in the back of the net. You're happy for him, hes's a big-time player and the most humble guy I've ever met."
The newest Timbers player, defender Liam Ridgewell, was pleased with his teammate's game-winning heroics.
"I like what I see!" laughed Ridgewell when asked about the midfielder. "If he can keep producing that every week, can walk out there and score a goal in 90 minutes, that's fine with me. It's a fantastic goal, it really got the crowd up, and that's a bit special."