SEATTLE — Last month, Sam Rogers turned 18. On Tuesday, the defender signed his first professional contract with Sounders FC 2. He went to school, got his yearbook and later that night, played with a Major League Soccer team.

Rogers, who is still finishing his senior year at Ballard High School, joined a group of nine S2 players who were signed to short-term agreements before the U.S. Open Cup game, which ended in a 2-1 Seattle win over Portland. After the fourth-round match at Starfire Sports Stadium in Tukwila, Seattle advanced to the Round of 16.

Even though Tuesday’s game somewhat functioned as a showcase for S2 players, such as Rogers, both of Seattle’s goals came from athletes already on the Sounders’ roster.

Zach Mathers, who hasn’t played for Seattle so far in the MLS regular season, and Aaron Kovar, who has just played 36 minutes for the Sounders this year, both scored for Seattle in the rivalry matchup.

After Portland’s Andre Lewis was called for a handball in the box, Mathers scored for Seattle on a penalty kick in the 55th minute, which was ultimately the game-winner.

“Kovar was talking me, he was like, ‘You seemed really confident,’ ” Mathers said. “I was like, ‘I did. I took penalties all the time when I was younger, so I feel very confident.’ “

Seattle began its attack quickly with a goal from Kovar, a Seattle native, in the third minute to give the Sounders an early lead.

“It was a pretty well-crafted goal,” Sounders coach Brian Schmetzer said. “That put a smile on my face.”

Portland’s Augustine Williams tied the game 1-1 in the 38th minute, but the Timbers didn’t score again.

Eleven of the teams’ combined 22 starters were S2 or Timbers 2 players who were signed to short-term agreements, which are contracts that last up to four days and allow athletes to play in nonleague games.

“I think they all played well,” Schmetzer said of the S2 players. “To me, what it says is that things that we’re trying to do at this club, to get the academy integrated, get S2 integrated with the first team — I think some of those things you’re seeing a little bit of, we’re bearing some fruit there.”

The Sounders were without Clint Dempsey, who was suspended from the tournament in 2015 for two years or six matches, whichever is longer, after ripping up a referee’s notebook. Jordan Morris was recently away on U.S. national-team duty and did not play on Tuesday, even though he is back in Seattle.

Defenders Joevin Jones and Roman Torres were also not available for the game as they are playing in World Cup qualifiers for their respective countries, Jones with Trinidad and Tobago and Torres with Panama.

These absences helped clear the way for less experienced players such as Kovar, Mathers and the members of the S2 team, to gain time on the field. Of the nine S2 players who Seattle signed to short-term agreements, six started against the Timbers.

“I was playing with a bunch of my S2 teammates, so it felt like we were just playing together,” Rogers said. “It was a lot more relaxing, and it was great to be out there with them.”