Endy Chavez, John LackeyJohn Lackey delivered a rare stinker Monday.

Lackey got roughed up by the Seattle Mariners as the Boston Red Sox suffered a 12-3 loss at Safeco Field. It looked like Lackey, who was knocked around for six runs in the fourth inning before being yanked, might have been battling a physical ailment, although the right-hander said after the game — his shortest outing since Sept. 9, 2011 — simply was the product of a bad day at the office.

“I felt pretty good the first couple innings,” said Lackey, after noting he felt fine physically. “The struggle obviously (came) in the fourth inning. I just wasn’t able to make a pitch to get out of there. It just kind of started going downhill and I couldn’t stop it.”

Lackey surrendered a home run to Logan Morrison in the second inning but otherwise looked sharp out of the gate. The right-hander’s fastball was up around 94-95 mph, and it appeared he’d hold up to his end of the bargain in what was expected to be a pitchers’ duel between the Red Sox’s red-hot starter and Mariners ace Felix Hernandez. Things completely unraveled in the fourth inning, though.

Lackey surrendered three straight hits with one out in the fourth. Robinson Cano started things with a ground-rule double, Kyle Seager tied the game with an RBI single and Morrison kept his foot on the gas with a base hit to set up runners at the corners. Mike Zunino walked to load the bases.

That’s when Dustin Ackley dug in for what proved to be the game’s most crucial at-bat. Ackley fouled off nine pitches and eventually pulled the 13th pitch of the at-bat to the right side. Mike Napoli made the play while ranging to his right and fired to second base in an attempt to start a double play. Stephen Drew couldn’t deliver a throw to first base, however, because Lackey never got over to cover the bag. Seager trotted home with the go-ahead run.

“I think in the midst of a 13-pitch at-bat he makes a quality pitch to finally record the ground ball, and then (I’m not sure) whether or not he anticipated the ball going through the hole,” Red Sox manager John Farrell said. “Nap makes a very good backhand play, gets the lead out, and just a lapse there didn’t allow John to receive the back-end of the throw on a potential double play.”

At the time, Lackey’s miscue seemed to be part-mental and part-physical. The veteran pitcher didn’t look right while running over to first base, and he could be seen stretching out during the fourth inning. Lackey even received a visit from Farrell and trainer Rick Jameyson immediately following Ackley’s ground ball, although, evidently, all was — and is — well on the physical front.

Lackey couldn’t record another out after Ackley’s lengthy at-bat. He walked Brad Miller and allowed a single to Willie Bloomquist to reload the bases, at which point Endy Chavez opened up the game with a bases-clearing triple over Brock Holt’s head in right field.

Chris Capuano surrendered five earned runs in 2 1/3 innings of relief, with Morrison, who went 4-for-4 with two homers, doing much of the damage for the Mariners’ offense.

Lackey had gone at least eight innings in three of his previous four starts before Monday’s implosion. He allowed three or fewer earned runs in six straight starts, and had gone at least five innings in a career-best 39 consecutive starts — the third-longest active streak in the American League behind Justin Verlander (41) and Jered Weaver (40).

The Red Sox can take solace in that Lackey is healthy despite getting beat up Monday. But Boston — a team struggling offensively — needs such stinkers to be few and far between in order to stay afloat in the American League East.