John Lackey was throwing everything he could at Seattle’s Dustin Ackley trying to get the out. After 13 pitches, the Boston righthander finally got the groundball he wanted and the chance at an inning-ending double play.

And then Lackey lapsed. He failed to get over to first base, letting the inning continue. And the Mariners took advantage, ending up with a six-run outburst.

“Just a lapse there didn’t allow John to receive the back end of the throw to get a potential double play,” Boston manager John Farrell said.

Lackey was knocked around for seven runs in his shortest outing since September 2011 and Boston was routed 12-3 by Seattle, the Red Sox fourth loss in five games.

Felix Hernandez threw seven strong innings, Logan Morrison hit two long home runs and the Mariners won their fourth straight after a weekend sweep in Kansas City.

Hernandez set a club record with his eighth straight start pitching at least seven innings and allowing two earned runs or less. He struck out six, walked none and efficiently handed the Red Sox their fourth loss in five games. Hernandez’s only miscues were David Ortiz’s RBI single in the first and Mike Napoli’s solo homer in the fourth.

Meanwhile, Lackey was done after 3.2 innings, snapping a string of 39 straight starts going at least five innings. He gave up seven hits and walked three. Lackey needed 36 pitches to get through three innings and threw 38 pitches in getting just two outs in the fourth.

Seattle’s big inning started with Robinson Cano’s double. Kyle Seager followed with an RBI single and Morrison was able to beat the Red Sox shift with a single into right field to put runners on the corners. Lackey was visibly miffed that Morrison’s grounder got through the defense and he followed by walking Mike Zunino to load the bases.

Then Ackley came to the plate. He fouled off pitch after pitch and finally hit a grounder to first. Napoli threw to second to get the second out but Lackey did not cover first base and the chance at an inning ending double play was missed. Seager scored and Lackey struggled with his composure. A wild pitch allowed Morrison to score and a walk to Brad Miller and Willie Bloomquist’s single loaded the bases.

Endy Chavez capped Lackey’s night by delivering the big blow and with one swing surpassing his RBI total for the season. Brock Holt was playing shallow and unable to chase down Chavez’s line drive, three-run triple giving Seattle a 7-2 lead.

Morrison had the first multihomer game of his career. His solo shot in the second inning off Lackey pulled Seattle even and he added to the offensive outburst with a tworun homer in the seventh, a 421-foot drive to center field off reliever Chris Capuano. Morrison finished with four hits for the third time in his career.

Notes — Hernandez allowed a homer for the first time since April 21 vs. Houston. … Lackey’s last start of less than five innings was May 14, 2013, vs. Tampa Bay when he went 4.1 innings. … Farrell said it’s possible injured OF Shane Victorino (hamstring) could rejoin the team this weekend in New York depending on his rehab starts this week.