There were fears the veteran Manchester United striker may not play again after suffering meniscus and cruciate ligament damage in April.
At the very least a lay-off of nine to 12 months was anticipated but the veteran forward defied those gloomy predictions to return to action on Saturday when he came on late on in the 4-1 win over Newcastle.
He said: "In my recovery I have not rushed anything. People may believe that because I have been out for six and a half months but I have been following protocol and my schedule day by day. The only secret has been working hard."
Ibrahimovic's one-year United deal expired when he was out injured and he was for a spell without a club before re-joining when it became clear he would recover. The Sweden international says he always expected to stay at Old Trafford although he admits he's not looking beyond this season at the moment when considering his long-term future.
The 36-year-old added: "It's fantastic to be playing. Jose said, 'Do whatever you want, I trust in you and you know what you have to do,' so I was not worried.
"The contract is all paperwork and I do the physical work. Even if I had a contract or not I needed to train. I knew I would stay because I wanted to stay. The numbers have never been an issue.
"I could have signed a deal after two months but didn't because I am honest with myself. I want to stay because I can perform. I am at the age where I can choose what I want to do. I don't sign five or 10-year deals. I am just happy to be back playing and doing what I am good at."