One Direction heartthrob Niall Horan is recovering from "major" knee surgery. Horan kept his fans in the dark about the operation but took to Twitter to tell them everything was fine after pictures surfaced of him hobbling around an airport on crutches, the Daily Mail reports.

"Yes I've had major knee surgery, a much larger surgery than first anticipated," Horan tweeted. "But I would like to tell u, it was a complete success! Considering the scale of it!"

"So all I ask of you guys is that you would respect my privacy as I try to recover and get for the where we are tour!" he added.

According to reports, the "Best Song Ever" singer suffered an injury last month and had to undergo a procedure to fix some torn ligaments. The group is slated to begin a South American tour in three months, at the end of April but Horan will have to take the next few months to recover.

One Direction will play at stadiums in the United States beginning this summer, Aug. 4 starting at the MetLife stadium in New Jersey. It's not clear what happened to Horan that led to the surgery but the singer has talked about his "dodgy knees" in the past.

According to the Daily Mail, he blames part of his knee troubles on a pesky squirrel.

"A squirrel attacked me. I got attacked by a squirrel in Battersea Park. They're dangerous. It's rare," he told The Sun in October 2012, the Daily Mail reports.

"I've torn most of the ligaments in my knee," he continued. "So no football for me. It's early retirement now. I've got a floating knee-cap!"

During an interview with contactmusic.com, Horan said that he doesn't think he will still be able to perform by the time he's 70-years-old because of his knees.

"[Mick] Jagger does it but I don't think at the moment I'd be able to," he said. "I've got dodgy knees."