The One Direction heartthrob 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.
He posted: "Amsterdam thank you for a great couple of shows! Appreciate you coming.
"Also guys! Can you limit what gets thrown on stage please! Coz somethin thrown at me tonight! Hit my knee! A lot of pain from it!
"Never wana sound like I'm complaining! But only having my operation 5 months ago, I still get a bit of pain and I'm very scared about it. (sic)"
Niall - who had major knee surgery in January - was seen limping around the stage in a video of the concert obtained by the Daily Mirror newspaper.
One devastated fan raged: "HE CANT BARELY WALK ITS SO SAD. (sic)"
The Irish singer's bandmates didn't escape unharmed either since poor Harry Styles was hit in the face by an unidentified flying object last night.
The 'Kiss You' hit-maker 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.