OAKLAND -- Through it all, Rays closer Diego Castillo felt fine.

Castillo was OK when he entered the game in the ninth inning to protect a three-run lead and began missing the strike zone. He stayed calm when the potential tying run reached scoring position. And he felt equal to the task when Matt Chapman, Oakland’s most dangerous hitter, stepped to the plate with two outs and slammed the second pitch he saw from Castillo over the Coliseum’s left-field wall for a three-run, walk-off homer that doomed the Rays to a crushing 5-4 loss.

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The setback lengthened the Rays’ losing streak to four games. They’ve also lost nine of their last 11 games. None of these numbers seemed reasonable to consider while Tampa Bay was scoring three runs in its half of the ninth to shatter a 1-1 tie.

In the bottom of the ninth inning, Oakland’s Robbie Grossman drew a leadoff walk from Castillo and advanced to second base on a wild pitch. One out later, Castillo walked Jurickson Profar but struck out pinch-hitter Beau Taylor. Then Oakland’s Marcus Semien extended his hitting streak to 17 games by singling up the middle to score Grossman. Center fielder Kevin Kiermaier’s accompanying throwing error advanced the runners. Any sense of impending euphoria that might have engulfed the Rays quickly evaporated.

Up came Chapman, who connected with a 1-0 slider.

“It looked like he had an approach where he was going to get a breaking ball and he wasn’t going to miss it,” Rays manager Kevin Cash said.

Said Castillo, “It was the pitch I was trying to throw. Obviously it wasn’t what I was expecting to happen, but that’s what happened.”

Cash was asked how a conceivably dominant reliever such as Castillo, who topped out at 98.6 mph on Thursday, can look so vulnerable.

“You’ve gotta throw it over the plate. You’ve gotta throw it over the plate,” Cash said, repeating one of baseball’s basic truths. “Big league baseball, you’ve gotta throw it over the plate. Believe me, we all know he’s trying to. He’s trying to execute pitches. It just didn’t happen tonight.”

What’s happened is that Castillo has yielded nine earned runs in his last 10 appearances spanning 9 1/3 innings. His ERA has soared from 1.50 to 3.51.

“I was out there ready to perform,” Castillo said. “But you’ve got to go out and work and come back and be ready next time.”

Rays starter Charlie Morton was ready for his outing. He performed his role as “the stopper,” allowing one run and four hits in 6 1/3 innings.

“I felt like their contact, as the game went on, got a little worse,” said Morton, whose ERA dipped to 2.31, which is second among qualified starters in the American League.

About five hours before Chapman’s walk-off homer, Cash sounded was confident Morton could be the guy to get the Rays back on track.

“We need something to go right for us,” Cash said. “I think we’ve got the right guy on the mound.”

With Morton pitching, his optimism was legitimate.

Even with the loss, Tampa Bay is 7-2 record in games Morton starts following a defeat. In an era when “openers” are beginning to supplant starters, nobody has settled upon a replacement for the “stopper” -- the starting rotation’s stalwart who takes the ball and doesn’t give it up until he has put his ballclub in position to end a losing streak. For the Rays, Morton was that guy, doing all he could to help keep them competitive in the opener of a four-game series.

The Rays still needed a dose of offense, and it finally came in the ninth inning, three innings after Tommy Pham’s 11th home run of the season had provided their lone run to that point.

Pham and Brandon Lowe coaxed walks from Oakland closer Blake Treinen before collaborating on a double steal. Avisail Garcia and Ji-Man Choi each stroked RBI singles. Garcia ultimately scored on Willy Adames’ sacrifice bunt to give Tampa Bay a three-run lead that would soon disappear.

Then came the A’s half of the ninth, and you know the rest.

“This game will punch you in the mouth,” Morton said. “The group we have, they’re awesome. We’re going to turn it around. I felt like we played really, really good baseball tonight.”