LAKELAND, Fla. — For the conundrum of what to do with a diminished Matt Harvey, the Mets still hold what once seemed to be an unthinkable option.

In theory, they could decide that the former ace is not among the top five pitchers in the rotation. Instead of carrying him on the Opening Day roster, they could leave him in extended spring training to build up his arm strength.

But in a 5-1 loss to the Tigers on Monday, Harvey hushed whispers of such a drastic action with perhaps the most encouraging outing of an otherwise tumultuous spring training.

“It’s definitely a positive move forward,” he said after allowing three runs and seven hits in 4 1⁄3 innings.

On paper, it hardly was a memorable performance. He lowered his ERA from 7.88 to 7.30. But from better command of his secondary pitches and a fleeting glimpse of Harvey’s old fastball, progress could be spotted beyond the boxscore. His fastball sat in the range of 93 to 94 mph and topped out at 96, an improvement from previous outings.

“Best stuff I’ve seen so far,” manager Terry Collins said. “We were looking at two big keys today. One was maintain his delivery and second was better command. And we saw both.”

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Yet Collins stopped short of declaring Harvey ready to challenge big-league hitters without the full force of his trademark fastball, the challenge he surely will face if the Mets bring him north.

“We don’t know until his next two outings,” Collins said. “We’ll see how he does, see how he continues to improve with the location of his pitches, because if he does, he’ll be fine.”

The Mets have yet to seriously consider leaving Harvey in extended spring training, a source said Monday. But with Opening Day 13 days away, it’s clear that the Mets have other viable options in Robert Gsellman, Seth Lugo and a suddenly resurgent Zack Wheeler.

Harvey, 27, is coming off his second major surgery. Last July, he had a rib removed to treat thoracic outlet syndrome, which deprived him of full feeling in his fingertips. He posted a 4.86 ERA in 17 starts in easily the worst season of his career.

The Mets know it might take until at least late May for Harvey’s fastball to come back for good. Of all the Mets’ bright young pitchers, he is coming off the most significant surgery.

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“Obviously, it’s kind of an unknown,” said Harvey, who insists he’s on track to be with the Mets on Opening Day.

He allowed a second-inning homer by James McCann, leaving a changeup on the inner half of the plate, the kind of lapse in command that he must work doubly hard to avoid without the leeway granted by fastball velocity. But he settled into a rhythm, retiring seven straight Tigers at one point.

But as he pitched deeper into the game, the Tigers pounced. Harvey surrendered hits to three of his final four batters, allowing a single on his 74th and final pitch.

Finishing strong, he believes, is only a matter of building up arm strength. He took heart in the smoothness of his mechanics and ease in which he ramped up his fastball. It was noted upon his return to the dugout. The flashing of 96 mph had not gone unnoticed.

“I guess everybody’s talking about that a little bit,” Harvey said. “And it was nice to go out there today and kind of dial it up a little bit into the mid-90s.”

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Notes & quotes: Closer Jeurys Familia returned to camp after playing for the Dominican Republic in the World Baseball Classic. He refused to discuss what could be a looming suspension from MLB for violating its policy on domestic violence . . . Fernando Salas allowed a hit in two-thirds of an inning in his first Grapefruit League game since acquiring his work visa. “You can tell he hasn’t been out there in a while,” Collins said of Salas, who pitched only twice for Mexico in the WBC.