The former Paris Saint-Germain striker secured a move to Major League Soccer side LA Galaxy last week after United agreed to cancel his contract three months early.
Ibrahimovic scored 28 goals in his first season at Old Trafford but struggled to recapture his best form after suffering cruciate knee ligament damage during a Europa League quarter-final against Anderlecht last April.
The 36-year-old Swede managed only one goal for United after making his comeback in November, with his final appearance for the club against Burnley on Boxing Day.
“For me, it’s always sad when the big players go in the direction of the end,” United manager Mourinho told a news conference on Friday.
“I remember for example Luis Figo, after his last match, with me and with Inter Milan.
“I always remember it as one of my saddest moments because I always feel very, very sorry when the big ones finish or go to the last period of their careers.
“Zlatan is a huge big player that European football is losing now and forever because he will not be back to this level.”
However, Mourinho added: “This period with Galaxy, these couple of years, will be very good for him and American football.”