Tyler O’Neill, left, greets teammate Enmanuel Valdez after Valdez hit a three-run home run in the third inning of the Red Sox’s 5-1 win over the Mariners on Sunday in Seattle. Lindsey Wasson/Associated Press

SEATTLE — Enmanuel Valdez hit a three-run homer, Garrett Whitlock pitched five effective innings and the Boston Red Sox beat the Seattle Mariners 5-1 on Sunday.

Valdez, a fill-in second baseman for the injured Vaughn Grissom, hit a drive to right in the fourth inning after starting the season 0 for 8.

“It was a huge momentum shift,” Whitlock said. “I mean, he lit us up in the dugout and that was just a ginormous swing.”

Tyler O’Neill connected for a solo shot in the second against Seattle’s Bryce Miller (0-1), who was charged with four runs and six hits in five innings.

Whitlock (1-0) helped Boston to a split of the four-game series after it dropped two straight one-run games. The big right-hander allowed three hits, struck out eight and walked none.

Whitlock relied mostly on his fastball after catcher Reese McGuire saw Seattle struggling at the plate and suggested a change in strategy.

“I feel like that’s something I need to continue to learn how to do,” Whitlock said. “And obviously that’s big on Reese to do. He the one that saw it and he’s the one that made the adjustments.”

Whitlock’s outing capped a strong series by Red Sox starters. They gave up five runs (four earned) in 22 innings with 27 strikeouts against five walks.

“Like I’ve been saying all along: I do believe we’re going to pitch,” Boston Manager Alex Cora said.

“It just kind of shows you what’s possible. You know that your rotation is very talented. We’ve been talking about these guys for a while, since we got here.”

Seattle got its only run when Mitch Haniger doubled and scored on Josh Rojas’ two-out single in the second.

Justin Slaten pitched 2 1/3 hitless innings for his first career save in his second major league appearance.

Coming off a strong rookie campaign, Miller struck out six and walked two.

The Mariners finished with four hits.

“Offensively, we just haven’t gotten it going yet,” Manager Scott Servais said. “Again, it’s four games into the season. We need to be more consistent there in what we’re doing, and we will be.”

WORTH NOTING

The Mariners signed right-hander Michael Mariot to a minor league deal and assigned the pitcher to Triple-A Tacoma. Mariot is 2-0 with a 5.85 ERA and two saves in 45 career major league appearances.

TRAINERS ROOM

Red Sox: Rafael Devers (shoulder) returned to the lineup after missing two games with a shoulder injury. The third baseman is a “full go,” Cora said. He went 1 for 5 with a single in the first and a strikeout.

Cora was coy about the status of closer Kenley Jansen, who woke up Friday with a sore back after earning a season-opening save in Boston’s 6-4 win over Seattle on Thursday. But Jansen warmed up in the ninth in case Slaten struggled, indicating treatment with a chiropractor was successful.