It was all the support he would need, and more.
Castillo and Andres Munoz combined on a shutout, Cal Raleigh hit a two-run homer and the Mariners won in their first postseason game since 2001, beating the Blue Jays 4-0.
Eugenio Suarez had two hits and two RBIs and rookie Julio Rodriguez reached base three times and scored twice for the Mariners, who can wrap up the series with a win in Game 2 Saturday.
The series winner plays AL West champion Houston in the Division Series starting Tuesday in Texas.
Suarez hit an RBI double off Blue Jays All-Star right-hander Alek Manoah in the first inning and Raleigh followed with a drive to right.
''It was very good going out there and having that lead,'' Castillo said through a translator. ''That gives me that little extra energy when I go on the mound.''
Throwing two different kinds of fastballs at 100 mph and his changeup at 92 mph, Castillo scattered six singles in 7 /13 innings. He struck out five and walked none, facing the second-highest scoring team in the AL.
''When you've got two pitches over 99 that are doing two different things, that makes it tough,'' Blue Jays infielder Whit Merrifield said.
Castillo, acquired in a midseason trade from Cincinnati, became the first pitcher in Mariners history to throw more than seven scoreless innings in a postseason start.
Raleigh said Castillo was ''awesome.'' Suarez called him ''unbelievable.''
''Credit to Luis, he was in total command today,'' Seattle manager Scott Servais said.
The right-hander turned away from home plate and pumped his fist after fanning designated hitter Danny Jansen to end the seventh, Castillo's third straight strikeout.
''Wow,'' Servais said. ''Some kind of performance by him.''
Castillo's only other postseason start came with Cincinnati in 2020, when he lost to Atlanta in the wild-card round. Castillo allowed one run in 5 1/3 innings in that one but the Reds were eliminated with a 5-0 defeat.
Toronto has lost four straight postseason games and seven of their past eight.
''It's two out of three,'' Manoah said. ''We'll be back tomorrow.''
Munoz came on in the eighth after Castillo hit George Springer on the left wrist with his 108th pitch. Springer went down in pain and was checked by the trainer but remained in the game. X-rays on Springer were negative, interim manager John Schneider said.
Munoz finished the eighth by getting Bo Bichette to fly out and retiring Vladimir Guerrero Jr. on a grounder to shortstop.
Matt Chapman's two-out double in the ninth was Toronto's seventh hit of the game, and first for extra bases. Munoz quickly closed out the Blue Jays.
Making his first career postseason start, Manoah gave up four runs in 5 2/3 innings, matching the total he allowed in six September starts. Three of the four hits off him were for extra bases.
''They beat me on my mistakes,'' a downcast Manoah said.
Manoah was in trouble right from the start, hitting Rodriguez on the hand with his fourth pitch of the game, and missing high and tight to Ty France with his fifth.
Rodriguez advanced on France's grounder and scored on Suarez's double to right. Raleigh followed with a 362-foot drive into the right field bullpen on a 3-2 fastball.
''An uncharacteristic first inning, to be sure,'' Schneider said.
Raleigh is the first player in Mariners history to homer in his first career postseason at bat.
''Cal's been on fire, not so much with hits but with homers,'' Servais said.
Last Friday, Raleigh had a pinch-hit home run with two outs in the bottom of the ninth inning to beat Oakland and clinch Seattle's first postseason berth since 2001.
The early outburst drained much of the energy and excitement from a sellout crowd of 47,402 on hand for Toronto's first home postseason game since the 2016 AL Championship Series.
Manoah hit Rodriguez for a second time in the fifth, and Seattle's star newcomer made Toronto pay once more, advancing to third on France's single to right and scoring on Suarez's grounder.
Springer and Bichette hit back-to-back two-out singles off Castillo in the third but Guerrero flied out.
Merrifield and Springer reached on two-out singles in the fifth but Castillo got Bichette to ground out.
Toronto was shut out for the sixth time in its postseason history, and the second time at home. It was the second time the Mariners have held an opponent off the scoreboard in the playoffs. They also did it Oct. 10, 2000, against the Yankees.
Rodriguez thought the inning was over when Teoscar Hernandez grounded out to short for the second out in the fourth, running almost all the way from center field to the infield before realizing his mistake. Chapman flied out to Rodriguez in center to end the inning.
EDDIE GETS 'EM READY
Former Blue Jays slugger Edwin Encarnacion threw out the first pitch. Encarnacion won the 2016 AL wild card game for Toronto with a home run in the 11th inning off Baltimore's Ubaldo Jimenez.
The Mariners will start LHP Robbie Ray (12-12, 3.71 ERA) in Saturday's Game 2. RHP Kevin Gausman (12-10, 3.35 ERA) starts for Toronto.
Ray won the 2021 AL Cy Young Award with the Blue Jays, going 13-7 with a 2.84 ERA and 248 strikeouts, then signed with Seattle. In his final five regular-season starts this season, Ray had a 5.27 ERA and allowed eight home runs in 27 1/3 innings.
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