Phillies top prospect Andrew Painter underwent Tommy John surgery on Tuesday

It seemed last week like a fait accompli, but Tuesday, it became a reality: Andrew Painter had Tommy John elbow surgery.

Painter, the Phillies’ top prospect and most important arm in the organization, underwent a reconstruction of the ulnar collateral ligament and ulnar nerve transposition surgery, the team said in a statement. Neal ElAttrache, the Los Angeles-based orthopedic surgeon who operated on Bryce Harper in November, performed the procedure.

The Phillies outlined a 15- to 18-month timeline for Painter to return to competition. The 20-year-old right-hander is expected to miss all of next season, president of baseball operations Dave Dombrowski said last week.

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“Normally [with] these surgeries, the players come back and do well for themselves,” Dombrowski said. “He’s still very young and has a long future ahead of him.”

Painter sprained the ulnar collateral ligament in spring training and experienced a recurrence of symptoms earlier this month. He had his throwing program suspended on July 4, and when the pain didn’t subside, the Phillies’ medical staff recommended surgery.

ElAttrache confirmed the diagnosis in a second opinion Monday.

Painter, a South Florida native, is expected to return this week to the Phillies’ facility in Clearwater, Fla., to begin his rehabilitation, the team said.

Before spring training, several Phillies officials believed Painter would claim the final spot in the rotation even though he hadn’t pitched above the double-A level. His bullpen sessions were spectacles, attracting even owner John Middleton. He was impressive in his lone start, touching 99 mph on March 1 against the Twins in Fort Myers.

Painter reported soreness in his elbow the following day. At the time, the Phillies’ medical staff and ElAttrache agreed that the injury might heal without surgery based on the degree of the sprain and its location on the proximal edge of the ligament.

The Phillies were even hopeful that Painter would make his major league debut later this summer.

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After not throwing for nearly six weeks, Painter began a gradual throwing program. Imaging showed some healing in the ligament, according to both Dombrowski and manager Rob Thomson. Painter was scheduled to face hitters in a live batting practice setting on July 4.

But when the pain resurfaced, the Phillies shut down Painter and ordered additional tests.

It’s unclear how Painter’s lengthy absence will impact the Phillies’ short- and long-term pitching plans. They were already looking to add rotation depth before the Aug. 1 trade deadline as insurance against injuries to their top five starters. Dombrowski has said the team is interested in re-signing Aaron Nola. With Painter out of the picture in 2024, will they be more aggressive in that pursuit?

“Of course it hurts the organization because he could have been a contributor for us [this year],” Dombrowski said. “But more importantly, just feel for him. But he’s a youngster. You start talking about missing next year and coming back when he’s 21, 22 years old. [Even] if there’s a little bit of time delay building his stuff back up, you’re still talking about a very young man who still has a long future ahead of him.”