The singer injured his knee during a gig for their 'Where We Are Tour' in Amsterdam, Holland and took to Twitter after the show to ask the band's over-zealous fans to stop hurling heavy items around during their concerts, reported Contactmusic.
"Amsterdam thank you for a great couple of shows! Appreciate you coming. Also guys! Can you limit what gets thrown on stage please! Because something thrown at me tonight! Hit my knee! A lot of pain from it," Horan tweeted.
Horan, who had major knee surgery in January, was seen limping around the stage in a video of the concert.
The Irish singer's band mates didn't escape unharmed either since poor Harry Styles was hit in the face by an unidentified flying object last night.
The 'Kiss You' hitmaker looked less than impressed when an item was chucked at his head, poking him in the eye, but barely missed a beat as he bounded around alongside Niall, Louis Tomlinson, Liam Payne and Zayn Malik.