After a collision near the end of the first half, Manneh left the field to receive treatment on a bruised quad, but promptly returned, slipping a close-range shot under Seattle goalkeeper Stefan Frei en route to a 1-0 Whitecaps win over the Sounders.
"I felt like I had a chance to go to score a goal," Manneh said. "I was still in pain, but I had to go take the chance."
Vancouver (11-8-13) won its third straight game and jumped a point ahead of Portland for the fifth and final Western Conference berth. Both teams have two games remaining.
The Sounders (19-10-3) remained tied with Los Angeles atop the overall MLS standings. The Galaxy visit FC Dallas on Sunday, then will face Seattle in a home-and-home series.
Manneh's goal, off a long pass from Pedro Morales, was the last action of the first half.
After scoring, the Whitecaps striker abruptly pulled up, waving off his teammates' attempts to celebrate with him, and limped away from the scene clutching his left hip.
He sat out the rest of the game with a contusion in his quadriceps, exacerbated by a sudden sprint on the goal so soon after his initial injury. After the match Manneh said he felt that he would be able to play in the Whitecaps' next game next week.
The Sounders defenders said they were surprised referee Jair Marrufo allowed Manneh back onto the field in a counterattacking situation. They felt it gave Vancouver an unfair advantage, as Seattle's Jalil Anibaba and Chad Marshall were left alone to defend.
"We were all under the impression that Manneh was still off the field," Anibaba said. "The next thing we knew, he has the ball and he's running in on goal."
Evans, who strayed from his usual position because he thought his direct opponent was off the field, said Marrufo should have waited until the play finished to allow Manneh back on.
"When we're pushing forward, there's always the risk of a counterattack," Evans said. "The referee has to wait until the play is done."
One year and one day before this match, Manneh scored a hat trick as Vancouver shocked Seattle, 4-1. At the time, Manneh was a relatively unknown 18-year-old who turned in a breakout performance in one of the most hostile away environments in the league.
Manneh said he remembered that night as he stepped onto the pitch at CenturyLink Field again 366 days later.
"I had a good feeling. It definitely gave me a lot of confidence coming into this game," Manneh said. "Everybody was like, 'You did it last year. Why not again this year?' Everybody believed in me in the locker-room, and I can see the support from the coaching staff and the senior players."
Vancouver travels to San Jose Oct. 18 before finishing the regular season at home Oct. 25 against the Colorado Rapids.
Whitecaps head coach Carl Robinson said that if his team wins its own matches, it shouldn't matter what the schedules are or what Portland does.
"(The win) puts us in a decent position," he said. "If we take care of our business, then we'll get to where we (want to) go. If we don't, we don't deserve to be there. It's irrelevant about what Portland do; we've got to take care of our business."
The win was Vancouver's first on the road in 10 attempts, since a 4-3 victory against the chasing Timbers on June 1. The Whitecaps also locked up the Cascadia Cup competition held annually among Seattle, Vancouver and Portland.
Vancouver punctuated the win with a series of clearances under pressure as the clock wound down, including a ball headed off the goal-line by Kendall Watson in stoppage time.
As the final whistle blew, Vancouver's players jumped off the bench and punched the air jubilantly, rushing onto the field to hug their teammates. Boos rained down from the stands, except from a small patch of Vancouver supporters in the upper deck who made the three-hour drive south.
"Not many people gave us the opportunity and believed that we could win, but I did, 1,250 (travelling) fans did and the group did, which was the most important thing," Robinson said. "It's all down to the players today, and I give them all the credit."