Ibrahimovic has seen a limited role in the first team since recovering from the knee injury he suffered late last season.
And with the side out of contention in Europe, Ibrahimovic felt it was best to make the move now instead of waiting until the end of the season.
The surgeon who operated on his knee, Freddie Fu, shared the following information.
Fu told Swedish outlet Expressen: "His idea was to try to go again and win more titles, especially the Champions League.
"When United went out to Sevilla, I think he felt it might be enough. It's my feeling. United can not catch up with City in the league either.
"They may be able to win the FA Cup, which is good, but not as big as the Champions League. He has, in principle, won everything else."