SEATTLE — Let’s start with what it wasn’t. The Mariners blew a four-run lead Thursday night against a struggling opponent when their bullpen threw away a fine first big-league start by Sam Gaviglio.

This could have been a disaster.

It wasn’t.

Guillermo Heredia’s two-out pinch RBI single in the ninth inning lifted the Mariners to a 5-4 walk-off victory over the Chicago White Sox at Safeco Field.

“That at-bat was important,” Heredia said, “but I went to the plate like it was a normal at-bat…and I got the job done. From the seventh inning on, I was in the cage hitting. So I was ready for that opportunity.”

Taylor Motter started the winning rally with a leadoff single to right against White Sox lefty Dan Jennings.

After Jarrod Dyson fouled off two bunts in attempting to sacrifice, he got one into fair territory — but right back to Jennings, who threw out Motter for a force at second.

“He threw me two sliders, man,” Dyson said. “I guess I was bailing out of the way. The last one, I was like, `Just hang in there no matter what. Just get to down.’”

Dyson’s speed paid off when he reached second on Chooch Ruiz’s grounder to third. The White Sox went around the horn for a possible double play, but Dyson beat the throw to second.

Chicago settled for an out at first base, which prompted an intentional walk to Jean Segura, whose three-run homer in the fifth inning boosted the Mariners to a 4-0 lead.

Heredia pinch-hit for Ben Gamel to provide a right-handed bat against Jennings (2-1) and punched a single to right. Dyson scored easily.

“It’s a tough spot for a young player,” manager Scott Servais said, “but he’s got a good swing. He’s not trying to hit homers or anything. Just have a good at-bat, and he did. He stayed inside a slider coming in on him.”

Nick Vincent (2-1) got the victory after pitching a scoreless ninth inning. The Mariners have won three of four since suffering a four-game sweep last weekend at Toronto. The White Sox lost for the 10th time in 12 games.

“The one thing that you want to make sure that you do when you’re having games like this,” Chicago manager Rick Renteria said, “is ultimately try to finish them up. That puts the icing on the cake.”

Gaviglio pitched five scoreless innings before handing a four-run lead to Casey Lawrence, whom the Mariners acquired May 11 from Toronto in a waiver claim.

The Mariners pulled Gaviglio after 75 pitches because of endurance concerns; he had thrown just two innings over the previous 13 days.

“It’s been exactly two weeks since my last start,” he said. “My pitch count was getting up there, and I just wanted to keep making good pitches.”

Lawrence worked scoreless sixth but yielded a two-run homer in the seventh to Matt Davidson, which cut the Mariners’ lead to 4-2.

The White Sox pulled even when Todd Frazier and Tim Anderson hit two-out homers in the eighth inning against Dan Altavilla.

That stuck Gaviglio with a no-decision and pulled Chicago starter Dylan Covey off the hook for a loss after allowing four runs in six innings.

Dyson gave the Mariners a 1-0 lead by turning on a first-pitch fastball from Covey in the third inning. The 360-foot drive into the right-field seats was Dyson’s second homer of the season — which matched a career high.

Next time up, Dyson ignited a three-run rally with a two-out single to left in the fifth inning. He then stole second and third, which gave him a league-leading 12 steals, before Covey walked Ruiz.

Segura followed with a three-run homer to left for a 4-0 lead that also extended his hitting streak to 17 games.