The 36-year-old former Sweden captain has been out of action since suffering cruciate ligament damage in a Europa League clash with Anderlecht in April.
"I said I will come back to finish what I started," Ibrahimovic, who signed a new one-year deal with United over the summer and is expected to be available before the end of the year, told Sky Sports News.
"Everything I build up from the first season, because obviously we won the three trophies... the ending for me was not the ending I wanted, or nobody wanted, especially after how the season went.
The injury had put Ibrahimovic's future in doubt, given he only had an initial one-year deal, but he says it spurred him on.
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"In that moment when it happened it was easier for me to say that I will come back because then I had a challenge," he said.
"And the challenge was that I never had a major injury, and all these people talking that 'it's over' or 'he's too old' -- all these doubts I had in my whole career.
Ibrahimovic made a big impact at United last season, scoring 28 goals in 46 appearances as the club won the Community Shield, League Cup and Europa League.
"I will not walk out limping.