The 19-year-old said that after the incident he is being careful about the injury, reported Daily Star online.
"A squirrel attacked me. So, I got attacked by a squirrel in Battersea Park. They're dangerous. It's rare," Horan said during a radio show. I've torn most of the ligaments in my knee. So no football for me. It's early retirement now. I've got a floating knee-cap," he said.
Horan had tweeted earlier about the pain he was suffering and that it might lead to an operation.
"You know the way I always had a bad knee? Well I added to it playing ball the other day! It is messed up now! Operation time," he tweeted.