At the very least a lay-off of nine to 12 months was anticipated but the 36-year-old former Sweden international defied those gloomy predictions to return to action on Saturday.

Ibrahimovic appeared as a 77th-minute substitute as Jose Mourinho's side ran out 4-1 winners over Newcastle at Old Trafford.

As if to underline his fitness, Ibrahimovic vaulted over a barrier to speak to reporters after the game.

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.

"Those close to me know what I have been doing - I was working five, six hours a day. When it happened I said to everybody giving up was not an option.

"My only focus was on coming back and coming back better. For that you need to work hard. I know when I'm back it's my head playing, not my knee. My knee has to follow.

"But this was not a one-man work, it was everyone's and it is thanks to them that I am back on the field and can play because if people really knew the real injury they would be in shock that I was even playing.

"But I keep that personal. People think it was only the ACL (anterior cruciate ligament) in my knee, but it was more than the ACL. But it's sensitive and that's why I don't choose to talk about it."

The uncertainty over Ibrahimovic's future was exacerbated when his contract with United lapsed during his lay-off. It was not until the new season had begun that a new one-year deal was signed but Ibrahimovic insists there were never doubts.

He said: "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."

Ibrahimovic, whose stellar career also includes stints at Juventus, both Milan clubs, Barcelona and Paris St Germain, has not looked beyond the end of the campaign.

He said: "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."