Ibrahimovic quickly became a fan favourite at Old Trafford following his 2016 arrival from Paris Saint-Germain, scoring 28 goals across all competitions before his season was cut short by a serious cruciate knee ligament injury in April 2017.
There was speculation at the time that the former Sweden international could be forced into retirement as a result of the injury, but United opted to re-sign him on a one-year deal and he made his comeback significantly ahead of schedule in November 2017.
However, the 36-year-old played just seven more games for United before being released to enable him to join Los Angeles Galaxy in March of this year, and he has now revealed that he turned down opportunities to play as he no longer thought that he could give his best.
"The situation was not about having a contract and earning money - it was the opposite. I said, 'I don't want a salary, you keep it'," Ibrahimovic told BBC Sport.
"When I was ready, I said to myself, 'I am not there'. They had a Zlatan before. After, I was not ready to be that Zlatan. I was selected in many of the games but I told the coach: 'Listen, I am not ready. I don't want to disappoint you.
"'The respect I have for my teammates and the coach. Select someone else who can do the job better'. I stood up and did it, even if I am Zlatan."
Ibrahimovic has scored 11 goals in just 14 MLS appearances since joining LA Galaxy.