SEATTLE -- There were numerous storylines developing for the Red Sox on Saturday night in what was shaping up as a satisfying, early-season victory.

From the strong pitching of Kutter Crawford to the tremendous plays on defense by Trevor Story and rookie Ceddanne Rafaela and some nice contact hitting from Pablo Reyes, the Sox seemed primed to clinch at least a split of this four-game series against a tough opponent in the Mariners.

But when it came time for Boston to close it out in the bottom of the 10th, closer Kenley Jansen wasn’t available due to back tightness.

That proved to be costly when a two-run lead slipped away. Young superstar Julio Rodríguez came through with a walk-off single to cap the three-run rally that gave Seattle a 4-3 win over the Red Sox.

Jansen woke up with discomfort in his back, but hoped he could get it loose in time to pitch. His back didn’t cooperate. That same ailment slowed the big righty late in Spring Training.

“Tried to come to the ballpark and get ready, but [the back] wouldn’t allow me to so I’ve got to be smart,” Jansen said. “If I go out there and aggravate it, it could be worse for a week or two so I’d rather lose one day or two.”

Without the man who has 421 career saves, manager Alex Cora turned to Joely Rodríguez.

The lefty had a tough night -- and also some tough luck. Luke Raley started Boston’s demise with a 62.2 mph bloop that found a patch of grass in short left field for an RBI single that sliced the deficit to 3-2. The only hard-hit ball the Mariners had the whole inning came next when Luis Urías hit a 104.7 mph seed that second baseman Enmanuel Valdez snared for the out.

A single to right by Josh Rojas was likely going to get Raley to third regardless, but it turned out to be worse when the ball went through the legs of right fielder Wilyer Abreu, putting the potential tying and winning runs in scoring position.

With the infield in, the Red Sox couldn’t prevent the tying run from scoring because J.P. Crawford simply hit the ball too soft (53.3 mph).

At that point, Cora went to Rule 5 rookie Justin Slaten for his Major League debut, and Julio Rodríguez found a way to get the job done, placing a 92.2 mph single into right to end it.

Why did Cora go to Joely Rodríguez -- who also struggled on Opening Day -- in that spot?

“That was a great pocket for him,” Cora said. “Jam shot over the third baseman. They put the ball in play and they found holes. Obviously, we would love to work ahead in the count. That would be better. But the quality of the contact wasn’t great. Bad luck.”

Jansen is also feeling like a victim of bad luck. He had no issues with his back when he saved Boston’s win on Opening Day. And when he went to sleep after not pitching in Friday’s loss, he had no inkling anything was wrong. The closer hopes he can pitch Sunday.

“It’s frustrating,” Jansen said. “You go to bed [fine], wake up in the morning, and have a bad back. You try to ramp it up and it’s not allowing you to, so we’ll see how it goes.”

Despite the loss, the Red Sox got a third straight strong performance out of the gate from a starting pitcher. Crawford went six innings, giving up three hits and just an unearned run, walking one and striking out seven. Starters Brayan Bello, Nick Pivetta and Crawford have compiled 17 innings while giving up 11 hits and three earned runs, walking one and striking out 19.

The starting staff's combined ERA of 1.59 is fifth best by Boston starting pitchers through three games in the past 30 years, and the best such tally since the club's World Series season in 2018.

Cora’s hope for the Red Sox to be surprise contenders this season hinges largely on his starting pitchers going five innings or more as much as possible.

The development of young position players like Rafaela will also help in that quest. The center fielder made a spectacular diving catch to rob Cal Raleigh in the bottom of the fourth.

In the top of the fifth, Rafaela led off with a double, sped to third on a flyout and scored on a fielder’s choice groundout by Reyes.

“The play was great,” Cora said. “The at-bats have been good. I think Pablo did outstanding for the role he played. It’s just a tough one. We’re pitching. We made two errors and it cost us the game.”