Less than two minutes after coming onto the field, Cooper converted a pass from Obafemi Martins in the 88th minute, and the Seattle Sounders came from behind to beat FC Dallas 2-1 and win their fifth straight game.
Martins also set up Lamar Neagle for the tying goal in the 62nd minute. He has an MLS-leading six assists.
Cooper subbed in for Neagle in the 86th minute. With Martins bringing the ball down the left side, Cooper ran into the box, took Martins' cross and one-timed it high into the left side of the net for his third goal of the season.
Cooper played his first four MLS seasons (2006-09) with Dallas, then returned last year after stops in New York and Portland. After drilling the ball past Dallas goalkeeper Raul Fernandez, he didn't engage in any elaborate goal celebration. The score was his 75th in MLS.
"I would never want to disrespect a former team that was good to me," Cooper said. "There's a lot of great quality on this (Seattle) team. I think we're all determined and hungry to get out there and contribute. Once I get that opportunity, I'm giving it my best."
Cooper was acquired in an offseason trade as coach Sigi Schmid and the Sounders sought to bolster their bench after a late-season nosedive in 2013. He has played in nine of Seattle's 10 games, coming off the bench in four.
Schmid said his reserves are well-prepared.
"They're good pros. They're motivated, and they're (ticked) off when they're not playing," he said. "When they get on the field, they want to make my life and my choices as hard as possible."
The Sounders (7-2-1) tied a club record with their fifth straight victory, extending their unbeaten streak to six and their league-leading point total to 22. They came from behind in five of the games during the unbeaten streak. Each of the five game-winning goals have come in the final 15 minutes.
"It's probably more nerve-wracking for us on the bench than it is for them on the field," Schmid said. "1-0 down for us is nothing. We can battle back from that."
Brazilian forward Michel gave FC Dallas (5-4-1) the early lead when he converted a penalty kick in the 16th minute. It was his fourth goal of the year.
Blas Perez helped set up the penalty kick when he charged into the box in pursuit of a ball, with Brad Evans and Zach Scott of the Sounders on him. In a tangle of feet, Perez went down, with Evans whistled for the foul.
FC Dallas, which started 4-0-1, lost its third straight.
"We deserved more, but that's the game," said Dallas coach Oscar Pareja.
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