Niall Horan issued a plea to One Direction concert-goers after the band's performance in Amsterdam on Wednesday.
The Irish guitar player, who underwent surgery on his knee in January, asked fans to be more careful about throwing items on stage as they are causing the boy's injuries.
"I honestly think that last night's crowd in Amsterdam was one of the loudest One Direction crowds yet! The place was rocking! Thanks, Holland," wrote Niall before adding, "Also guys!
Can you limit what gets thrown on stage, please? Coz something was thrown at me tonight! Hit my knee! A lot of pain from it!
"Never want to sound like I'm complaining!" added Niall. "But only having my operation 5 months ago, I still get a bit of pain and I'm very scared about it."
One Direction's Liam Payne and Niall Horan take a tour break at WimbledonNiall was left using crutches at the beginning of the year after having surgery on his left knee.
The 20-year-old had tried to keep news of the procedure quiet, but after pictures emerged of him hobbling around with a support on his leg, Niall sought to reassure his fans about his health.
"Hey guys I know most of you have seen those pics today!" he wrote on Twitter. "Yes I've had major knee surgery, a much larger surgery than first anticipated.
"But I would like to tell u, it was a complete success!" he added. "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!"