Ronaldo returned to United this summer but there was a chance to bring him back to Old Trafford in 2018, while he was playing for Real Madrid.
Mourinho and Ronaldo previously worked together for three years in the Spanish capital and Jim White of talkSPORT has claimed former vice chairman Ed Woodward wanted to broker a deal.
White explained: "Over the years there have been many discussions over Ronaldo’s return to Manchester United.
"It might have happened when he finished at Real Madrid, when he was still in his pomp.
"But it wasn’t possible because Jose Mourinho made it clear he at that time had other priorities."
That summer instead saw Ronaldo move to Juventus, where he stayed for three years before returning to England in August.
Mourinho, meanwhile, had just taken United to second in the Premier League table and was desperate for the club to sign a central defender. Toby Alderweireld and Diego Godin, as was Jerome Boateng.
It's potentially the reason Mourinho opted against a Ronaldo move.
Instead, United signed Fred and Diogo Dalot and a poor start to the season saw Mourinho sacked that December.
Ronaldo's return has personally been a solid one, with the five-time Ballon d'Or winner scoring 24 goals in all competitions.
United finished sixth however and failed to qualify for the Champions League with a points tally of 58 - their lowest-ever in the Premier League.
Six first-team players left at the end of the season and new manager Erik ten Hag looks set to complete his first signing.
A deal has been agreed for Feyenoord full-back Tyrell Malacia, while negotiations continue with Barcelona for Frenkie de Jong.
The 25-year-old worked under ten Hag at Ajax and will play a vital role in United's new system.
Ten Hag took his first training session on Monday and his philosophy was on show.
He worked with a number of academy graduates and stressed he wanted a high tempo on the pitch.
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