SEATTLE – Just hours before he was scheduled to start for the Mariners in the first game of their Thursday doubleheader in San Diego, right-hander Taijuan Walker instead was traded to the Blue Jays for a player to be named or cash considerations.

Only players currently in teams’ 60-player pools can be traded under the current COVID-19 agreement, so prospects not in camp can only be included in trades as players to be named later.

Walker, 28, was regarded as one of the better starting pitchers available prior to Monday’s Trade Deadline, given that he’s a pending free agent at the end of this season and Seattle clearly is in seller’s mode as a rebuilding team with a 12-19 record in the American League West.

Manager Scott Servais said Walker was a big part of the development of young starters Justus Sheffield, Justin Dunn and Nick Margevicius this season as well as helping first-year pitching coach Pete Woodworth and would love to see him back with the Mariners at some point.

“You talk about maturing as a pitcher and a person, what he was able to do in our clubhouse, let alone what he was able to do on the field and how he’s grown there,” Servais said of Walker. “I’m happy for Taijuan to get this opportunity to continue a really good season. This is his free-agent year. I’d love to see him go out and dominate the rest of this season and come back and sign with us going forward.

“It’s a very good environment for him to continue to grow and produce. Selfishly, I just love having him in the clubhouse and what he was able to bring there from a leadership standpoint.”

Rookie Ljay Newsome got the start in Walker’s place on Thursday, but Servais said it hadn’t been determined yet how the spot would be filled going forward. The Mariners have been going with a six-man rotation for the first half of the season, but they have a number of off-days in September that could allow them to go with a five-man rotation while still getting the same workload.

Walker went 2-2 with a 4.00 ERA in five starts for Seattle after re-signing with the Mariners this spring on a one-year, $2 million contract as he looked to rebuild his value after spending most of the past two seasons working back from Tommy John surgery and a subsequent shoulder issue while with the D-backs.

After a rough season debut, Walker is 2-1 with a 2.66 ERA and 24 strikeouts in 23 2/3 innings over his last four starts, holding opposing batters to a .165 average. His two wins came against the A’s and Dodgers, including seven scoreless innings on one hit against Oakland. He gave up three solo homers in the first three innings in his last outing against the Dodgers, but finished up with four hitless frames in a 6-4 win.

The deal is the second between the two teams in the last five days, as designated hitter Daniel Vogelbach was traded to the Blue Jays on Sunday in exchange for cash.

Walker originally was drafted by the Mariners in the first round (43rd overall pick) in 2010 out of Yucaipa High in California and went 22-22 with a 4.18 ERA In 65 games (62 starts) from 2013-16 before being traded by general manager Jerry Dipoto to the D-backs along with infielder Ketel Marte for shortstop Jean Segura, right fielder Mitch Haniger and reliever Zac Curtis.

He was 9-9 with a 3.47 ERA in 32 starts for Arizona before undergoing Tommy John surgery on April 25, 2018. But the 6-foot-4, 235-pounder has appeared fully healthy this season in his return to Seattle and now has a chance to help the Blue Jays make a playoff push as they enter play Thursday at 15-14 and currently hold the No. 8 postseason position in the AL standings.