SEATTLE — Luis Castillo’s smile was small and tight as he heard Seattle Mariners manager Dan Wilson tell him on the mound that he was being replaced. It was the third inning of Game 4 of the American League Championship Series, with the Mariners trying to hold off the Toronto Blue Jays, who had already won one game on the Mariners’ home field. Castillo, who had thrown fewer pitches than he had in any of his 247 starts in the major leagues, nodded in assent — if not agreement — and handed the ball to his manager.

What happened before Wilson’s decision was bad; what occurred after was worse. The Mariners’ relievers failed to contain the Toronto offense, in an 8-2 loss, and with the series tied at two games apiece, Seattle will go into Game 5 with its pitching options even more complicated by how Wilson’s choices played out.

Every postseason decision is evaluated through the prism of the result, which is not always fair but is October reality.

“You make decisions,” Wilson said after the game, “and you have to live with them.”

What the Mariners’ staff had talked about going into Game 4, Wilson explained, was that Seattle wanted to be aggressive in going to the bullpen. When Toronto blew out the Mariners in Game 3, Wilson was able to hold back all of his best relievers: Gabe Speier, Matt Brash and Andres Munoz. Additionally, Bryan Woo — who had been Seattle’s best pitcher before getting hurt Sept. 20 — would be available out of the bullpen, if Wilson found a suitable opportunity.

Castillo is a three-time All-Star, the most accomplished of the Mariners’ vaunted rotation of starting pitchers, known for his sturdy reliability. He has also had a year of diminished stuff, with his swing-and-miss rate the lowest of his career; Toronto had scored eight runs in 10 innings against him during the regular season.

In the first two innings Thursday, Castillo threw crisply, attacking the strike zone with a fastball that reached 95 mph. But in the third inning, everything changed. Isiah Kiner-Falefa pulled a double down the third-base line, and with a 3-2 count, Andres Gimenez pulled a slider into the right-field stands, giving the Jays a 2-1 lead. Relievers began stirring in the Seattle bullpen, and as Nathan Lukes and Vladimir Guerrero Jr. singled, Speier quickly warmed up. Too many pitches in the middle, Cal Raleigh would say later.

Alejandro Kirk drew a walk to load the bases. Wilson’s options in that moment: stick with Castillo, or summon Speier, a lefty, to face left-handed hitting Daulton Varsho.

Wilson emerged from the dugout and didn’t immediately signal to the bullpen; rather, he spoke to Castillo, telling him he was intending to call on Speier. Castillo had thrown only 48 pitches, the fewest in his nine-year career. He returned to the dugout and watched the rest of the inning play out from the top step.

“It’s a tough decision,” Wilson said, “and it was not an easy one to tell him. But that’s what we went with.”

Varsho fell behind 1-2 in the count, but he fouled off two pitches and worked the count full before drawing a walk. Toronto’s lead was 3-1. Speier struck out the next two hitters to avoid further trouble, and Castillo met him with a high-five as he stepped into the dugout.

In the next inning, Speier pitched himself into trouble again. After a Kiner-Falefa single and a sacrifice bunt, right-handed hitting George Springer batted next.

Wilson had three options in that moment:

He could have summoned Brash, his best set-up man, to face Springer. He could have effectively compelled Speier to pitch around Springer. Or, with the left-handed hitting Lukes on deck and Guerrero set to follow, he could allow Speier to face Springer.

Wilson went with the third option, and Springer ripped a double into the left-field corner, extending the Jays’ lead to 4-1. By the time Lukes grounded out, Speier — the best left-handed option in the Seattle bullpen — had thrown 32 pitches, more than any outing in his career.

Wilson summoned Brash into the game with the Mariners down by three runs — probably not the situation the manager envisioned at a time when his team was leading the series. Seattle’s bullpen had to cover 20 outs Thursday.

Bryce Miller starts Friday, in the Mariners’ final chance to win a championship series game in front of their home crowd. This series is guaranteed to return to Toronto — but whether it does with a Seattle lead is up to Miller and a bullpen that was used heavily in Thursday’s loss.

“We did use bullpen guys tonight, but they were very well rested again,” Wilson said. “So I think that we’re still in good shape in terms of our bullpen and also, we have Bryan down there as well, and we’ll utilize him when the time is right.”

It is unclear how Castillo felt about all of this. In his time with the Mariners, he has been known for consistently sticking around to answer questions after his starts, good or bad. But by the time reporters were permitted into the Mariners’ clubhouse after Game 4, Castillo was gone.