The 37-year-old Swede has scored 46 goals in 49 games since moving to the Major League Soccer side in March 2018.
“I could play easy in the Premier League, so if United needs me, I’m here,” he said.
Ibrahimovic scored 28 goals in his 2016-17 debut season for United before suffering a knee injury.
His two-year contract was terminated early by mutual consent to allow him to move to the United States.
However, if he were to rejoin United, he would not be allowed to play for them until January, when the English transfer window reopens.
He signed a new deal to stay with Galaxy for the 2019 campaign in December.
“Galaxy has me, so I’m sorry,” he added.
“I did my job in Europe. I enjoyed it, I have 33 trophies that I brought with me here and hopefully I can get something here.
“And then we will see where that adventure finishes.”
Ibrahimovic added that he still watches Manchester United’s games and insisted they were unlucky in their home defeat by Crystal Palace on Saturday.
Marcus Rashford missed a penalty in the loss, just as team-mate Paul Pogba had done in the draw at Wolves five days earlier.
“I saw the last game and I think they were unlucky,” he said.
“If they score the penalty it’s a different game, but a game in England is not finished until it’s finished.
“Anything can happen, especially in the last minutes where everything is in the heat of the moment.”
Ibrahimovic also revealed that he is still in touch with Pogba, who has been linked with a move away from United this summer.
“I speak with him. A lot of advice I give – but nothing I share with you!” Ibrahimovic added.