HOUSTON — Michael Conforto, the brilliant young outfielder whom the Mets regard as a franchise cornerstone, will undergo surgery sometime this week to tighten a torn capsule in his left shoulder.

After seeking a second opinion, Conforto decided to go outside the organization, with Dodgers team physician Neal ElAttrache scheduled to perform the procedure. Mets assistant general manager John Ricco declined to offer a projected timeline for a return.

“It’s going to be months,” Ricco said Saturday. “Until he has the surgery, I don’t think we’re ready to put a timeframe on it, but it’s going to be a considerable rehab. Again, I don’t want to speculate until he actually has it, they go in there and they see what the surgery actually entails. We’ll have a better idea after that.”

Conforto, 24, was in the midst of a breakout season. One year after his struggles landed him in the minor leagues, he hit .279 with 27 homers to earn his first trip to the All-Star Game. But one fateful swing-and-miss on Aug. 24 ended his season and ushered in questions about whether he’ll ever regain his form.

Conforto dislocated his left shoulder and an MRI revealed he had torn his shoulder capsule or the connective tissue that helps provide stability in the joint.

Dr. Armin Tehrany, a Manhattan-based shoulder expert, recently told Newsday that surgery could cost Conforto four to six months. But during such procedures, it’s common for surgeons to examine the rest of the shoulder for any other damage that can’t be seen with MRI exams, adding a degree of uncertainty to any timeline.

Said Ricco: “It was described to me as they need to tighten up the capsule so the likelihood of it popping out again is diminished.”

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For the Mets, the injury could represent the first major body blow in a rapid rebuilding effort that has barely begun. They already face questions at third base, second base and centerfield, and a lengthy recovery by Conforto also could mean shopping for another corner outfielder in the offseason.

“I think we have some time to kind of talk about that internally,” Ricco said. “First things first: We’ll get through the surgery and see what the doctors say. That certainly will play a role, but we have some time here before we head into full offseason mode.”

With David Lennon