U.TV reports, The Irish star was spotted wearing a cast on his right foot during the group's gig in Montreal, Canada on Saturday night, but he has no idea how he got injured.
Asked about the obvious injury onstage by bandmate Louis Tomlinson, the 21-year-old said: ''I woke up before the show the other day and I had a pain in my foot. Then I went to get an X-ray today and I've got a fractured right foot and I don't know how."
The latest setback comes as Horan was recovering nicely from knee surgery earlier this year.
In a post on Instagram in July, Horan wrote, "First time I've been able to touch my a** with my heel since I got the operation. Sounds so dumb, but it's a big day in my head."