The former Sweden striker was United's top scorer last season, hitting 28 goals as they won the Europa League, the League Cup and the Community Shield.
Ibrahimovic's one-year deal at Old Trafford expired in June and was not extended following the injury.
United begin the new league season with a home clash against West Ham United on Sunday, with Mourinho hopeful of getting off to a good start as they look to improve on last term's disappointing sixth-placed finish.
"Zlatan is injured and needs time to recover", Mourinho said on Friday.
"Now we're in the Premier League, not only do I get to play at the highest level of football, but I continue to be part of a team that achieved something special and we will continue on that journey together to try and achieve much more".
The Mexico worldwide, nicknamed Chicharito, enjoyed a productive four-year spell at United from 2010, scoring 59 goals in more than 150 appearances as the club won two league titles and reached the 2011 Champions League final.
The big money arrivals of Romelu Lukaku, Victor Lindelof and Nemanja Matic certainly represent the necessary statement of intent, though United have tried and failed, often at great expense, to find a winning formula since Sir Alex Ferguson retired.
Mourinho admits it was a challenge to keep his players fired up to compete for Europe's secondary competition but he does not expect any problems this year.
"We have added three players, one defender, one midfielder and one attacking player", the Portuguese boss explained. Phil Jones and Chris Smalling have been linked with moves away from the club in recent weeks - while Marcos Rojo remains sidelined with a knee injury.
"I just think he was very clear showing what he did past year was not enough for him. At the same time, let's make a decision that will make us all happy". "I think we are ready".
"I am not a hard person to work with, and I understand the reality of the market".
"So I don't think he'll be a huge loss to them".
The focus may have been on Manchester last season, but the title went to the capital and the run-in was a battle between two London clubs.
"What I had in other clubs was more players scoring goals, what we had was midfielders, wingers scoring, central defenders scoring goals at set-pieces", Mourinho (pictured right) said. Ibrahimović's goals got the headlines and secured many results past year, but it was Pogba that made the team tick. Obviously the [remaining] 25% would be a player for another position playing from the sides.