According to Italy's Sky Sport (via Sky Sports News), the Swedish striker was released because of an injury but has continued his rehabilitation at United's Carrington training complex, and the Red Devils are now "stepping up their efforts" to secure the 35-year-old's return.
United signed Ibrahimovic on a one-year deal last summer, and he enjoyed an excellent debut season in England as he netted 28 goals in 46 appearances—17 in the Premier League.
However, he picked up a cruciate ligament injury in a UEFA Europa League clash with Anderlecht in April, and United subsequently decided not to take up the option to extend his contract by a season as he was expected to be absent until early 2018, per The Guardian.
Per Sky Sports News, the former Ajax, Juventus, Inter Milan and Barcelona star has been linked with a return to AC Milan this summer, while Atletico Madrid and LA Galaxy were also reported as interested.
However, no deal has yet been struck for Ibrahimovic to join a new club, in part likely because few clubs would be willing to commit to a 35-year-old forward—36 in October—with such a serious injury.
Ibrahimovic played a huge role for Jose Mourinho's United last season, though, and he could prove a major asset in 2017-18 should he make a full recovery.
Compromises will likely need to be made as he will no longer be the automatic No. 1 striker after the summer signing of Romelu Lukaku.
But, given it would be a deal that should benefit all parties involved, it would be no surprise to see Ibrahimovic pull on a United shirt again in the new campaign.