The Swede, who is expected to miss the rest of the year with his injury, was pictured being driven into United's Carrington facility on Wednesday.
The 35-year-old had initially joined United on a one-year deal last season, but it was expected that he would stay at the club for an additional year.
However, the Red Devils decided against exercising the option of one more year after Ibrahimovic suffered a serious knee injury in a Europa League game in April.
Despite his injury, Ibrahimovic ended the season as United's top scorer with 28 goals in all competitions.
Zlatan Ibrahimovic returns to Carrington to continue recovery despite being a free agent. (Mirror) pic.twitter.com/mz0ZcwbLje"
— Red Army Exclusives (@chrisjec74) July 5, 2017