"I could have gone down the road of not saying anything to Billy, that the knee was feeling fine and got a contract," Piper told the Nottingham Evening Post on Friday.
"But the problems were always going to be there and I don't want to be someone just sitting in the stands picking up money.
"The club have looked after me brilliantly the times I have been there and I would never lie to anyone, especially Billy, who has been so good to me.
"I have got two little boys to consider as well because they are the most important thing to me.
"I want to still be able to run around with them in the park, rather than jeopardising my health by carrying on playing.
"The first few days it was not too bad but over the weekend it was really hurting.
"At least now I can look back and know I gave it everything I had and it just didn't work out. I'll have no regrets."