CARSON, Calif. — Earlier this season, there were times when Sounders forward Jordan Morris sincerely doubted whether he belonged on the field.

He would eye the clock with the zeal of high schooler on the last day before summer vacation, willing it upward toward 90.

“I was almost waiting for the whistle,” Morris said. “I’m like, ‘I’m too nervous to be out here. I don’t know what was going on.’ “

That timid, gun-shy rookie bore little resemblance to the attacking terror that led the way to the defining win of Seattle’s season Sunday afternoon at the StubHub Center.

Morris scored twice in a 10-minute span of the second half, the go-ahead goal plus an insurance tally, as the Sounders stunned the Galaxy 4-2 for their first win in this stadium since 2009.

The Sounders (11-13-5) are within three points of rival Portland for the final playoff berth — with two matches in hand, one of which is Wednesday against last-place Chicago.

Though interim coach Brian Schmetzer stuck to the usual it’s-a-team-sport spiel he usually reserves for big games from Nicolas Lodeiro or Clint Dempsey, his teammates were less restrained.

“He’s been good all season,” said Osvaldo Alonso, who netted Seattle’s fourth goal with a long-range strike. “But today, playing against a big team, it’s amazing. I’m very happy for him.”

The second goal said more about the growth of Morris’ skill set.

He’d been irrepressible for most of the second half, galloping behind the defense. He set the chance up and buried it, a well-place cross headed down by Alvaro Fernandez for Nelson Valdez, who was denied at close range.

Morris gathered the rebound, took a touch to gather his composure and finished with his left foot — his 12th goal of the season, most in MLS history by an American rookie.

“As the season has gone on, the game has slowed down for me a little bit,” Morris said. “In moments like that, I don’t panic as much. I took the touch, and maybe earlier in the season, I would have hit it first-time.”

The earlier game-winner was more illustrative of the development of Morris’ mindset, about the steps he’s taken since that crisis of confidence.

Morris is almost certainly going to win MLS Rookie of the Year. On paper, he’s done more than his share to contribute to the Sounders’ attack. But for many, the missed chances in vital moments were the ones that stuck out.

Think of the Houston match, when he came off the bench as a break-glass-in-case-of-emergency sub with the game there for the taking. Played into the clear by Lodeiro in the decisive moment of a tied game, Morris missed. The Dynamo scored shortly afterward, and though Lodeiro answered in stoppage time to rescue a point, two were dropped.

Those were the moments that dogged him, the ones that ate at the young player who has admitted that the criticism bothered him.

Morris said those earlier misses were absent from his mind when he picked Dan Steres’ pocket deep in Galaxy territory in the 63rd minute, but “I’m sure after the game, if I’d have missed that chance, I’d have been thinking about it a bunch.”

The game was balanced on tenuous ground. Robbie Keane had staked the hosts to an early lead, only for Jelle Van Damme’s own goal to tie it up shortly before halftime.

If anything, the match had been tilted in L.A.’s direction before Morris’ go-ahead goal. Certainly, most of the chances had been created at the other end of the field.

“It could have gone either way,” Schmetzer admitted, and that was no small thing, given the stage of the season and Seattle’s standing in the Western Conference.

Morris broke free, bursting into the right side of the box.

“I know you guys don’t like when I use the outside of my right (foot),” Morris joked, a dig at his critics, but that style of striking the ball comes more naturally to him.

He hit the ball cleanly, a screwball that swerved around Brian Rowe into the upper, left-hand corner of the net. And the Sounders were on their way to a huge win, unlikely until the moment it left his foot.

“The game could have changed in that moment,” Schmetzer said. “Big-time players come up in those moments and make a difference in games. Jordan’s performance was a big-time performance tonight.”