The Irish singer had an operation in the U.S. to correct a long-standing problem with his left knee ahead of the band's next tour in May (14).
On Thursday (16Jan14) he was pictured for the first time since undergoing the procedure as he arrived at London's Heathrow Airport.
Horan was seen hobbling on crutches as he left the arrivals terminal and headed to a waiting car.
He later took to Twitter.com to inform fans he had undergone "major" surgery but assured them he is on the mend.
In a series of posts, he writes, "Hey guys I know most of you have seen those pics today! Yes I've had major knee surgery, a much larger surgery than first anticipated. But I would like to tell u (sic), 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 (ready) for the Where We Are tour!"