Just before the start of the season’s second half, general manager Sandy Alderson said on Friday that the Mets will have to play “exceedingly well” for him to alter his plan to approach the July 31 trade deadline as a seller.

But while the Mets remain open to trading their veterans, Alderson set clear parameters for what appears to be a looming sell-off. He also doubled down on a longstanding team-building philosophy that places a higher priority on offense than defense.

Alderson insisted that his team’s valuable young starting pitchers — such as righthander Jacob deGrom — are unlikely to be moved.

“This is not a tear-down situation,” Alderson said. “This is what I believe is a sort of a pause button. We’re going to have a lot of players that are free agents at the end of the year, a lot of payroll that will become available, and so we’re not looking to rebuild.”

Reliever Addison Reed likely will generate plenty of interest leading up to the non-waiver trade deadline. Outfielders Jay Bruce and Curtis Granderson and first baseman Lucas Duda also could bring back some value. All are veterans on expiring deals — the only types of players the Mets are likely to move because they believe that contention remains within reach next season.

“We’re looking to make sure that we have the nucleus of a competitive team going into next year,” Alderson said. “As I said, while we wouldn’t completely discount any possibility of a trade of that type because who knows what may be presented, I don’t think enough would be presented for us to bite on that.”

Of course, the Cubs recently sent shock waves through the industry, acquiring ace Jose Quintana from the crosstown White Sox for a package of four prospects headed by the highly regarded Eloy Jimenez. Like deGrom, Quintana is a former All-Star under control potentially through 2020.

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Alderson said no such deals have been proposed to the Mets. “That’s a possibility only because you never quite know what’s going to be presented,” he said. “But I say that sort of trade is exceedingly unlikely.”

On Friday night, the Mets began a stretch of games against the Rockies, Cardinals and A’s, teams they potentially could make up ground against. Still, the climb will be steep. The Mets (39-47) entered the second half 10 1⁄2 games behind in the race for the second wild card and with less than a 10 percent chance of making the playoffs.

Part of that failure has been traced to a pitching staff that has sorely missed ace Noah Syndergaard and closer Jeurys Familia. A diminished staff that owns a 4.94 ERA (28th in MLB) has exposed what has emerged as a critical weakness: defense.

Asdrubal Cabrera’s spotty defense at shortstop has prompted Jose Reyes’ move to that position, though he also lacks range at this stage in his career.

But when it comes to run prevention, Alderson repeated his longstanding belief that pitching constitutes a larger share of the burden than defense. Indeed, the Mets made that equation work in 2015, winning the National League pennant behind a dominant pitching staff that shielded a range-deficient defense.

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“What really is important on that side of the ball is pitching, and our pitching has been lousy,” Alderson said. “And it’s partly because of injuries and it’s partly because of performance otherwise.”

The comments came a few days after pitching coach Dan Warthen pinned some of the pitching staff’s struggles on a defense that ranks near the bottom of the National League in various defensive metrics.

Alderson acknowledged that part of the retooling for next season will include improving the Mets’ defense. But despite this season’s struggles in the field, he will not be rethinking his offense-first philosophy.

“In terms of having a bias toward offense, I think that will probably always exist with me,” he said. “And I think it exists pretty much across the board with modest adjustments from person to person or team to team.”

Alderson said he would not be opposed to making a deal with the Yankees. The clubs have not made a player-for-player swap since the Mets sent Mike Stanton to the Yankees for Felix Heredia in 2004.

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“It’s certainly something I would consider, yes,” he said.

Alderson also hinted that shortstop Amed Rosario and first baseman Dominic Smith will not be promoted until the Mets move veterans who already are occupying their positions. He called it an “oversimplification” to assume that promoting Rosario would on its own solidify the Mets’ shaky defense.

“We want Rosario, we want Smith to come up under the best possible circumstances,” he said. “And right now, we think the best thing for both of them is to continue to play in Las Vegas until the situation here clarifies one way or the other.”