Ronaldo departed Real Madrid ahead of the 2018-19 campaign after a nine-year stay, joining Italian champions Juventus in a shock transfer. Los Blancos signed the Portugal international from United in 2009.
And according to Simon Mullock of the Sunday Mirror, Mourinho was not keen on having Ronaldo join the Red Devils for a second time.
"The United manager felt that handing Madrid a £90 million fee for a player who would also command wages of £500,000 a week made no sense when he had made it his top priority to sign defensive reinforcements," Mullock said. "But Mourinho's stance backfired when United chiefs then failed to deliver Toby Alderweireld, Harry Maguire or deadline-day target Diego Godin."
While supporters no doubt would have loved to see a goalscorer of Ronaldo's pedigree rock up at United, the defensive issues in this team are there for all to see.
Although Ronaldo won't be donning a United shirt anytime soon, supporters will see him at Old Trafford again on October 23, as the two teams drew each other in the UEFA Champions League. Squawka Football summed up why the Portuguese is a legend at United:
As we can see, the 33-year-old has tended to fare pretty well up against his former club:
Ronaldo had tremendous success at Madrid, becoming the Spanish club's all-time record goalscorer in his nine years there. In addition, the forward also won the Champions League on four occasions.
While United do have a lot of attacking options in their squad, someone with that goalscoring prowess and experience would have made a big difference to the team.
The concerns about the deal make sense too. Despite Ronaldo's immense physical condition he is heading towards the twilight years of his career, and paying such a massive transfer fee would have represented something of a gamble.
Ronaldo will no doubt make big contributions for Juventus in Serie A and the Champions League, but he's been frustrated in front of goal during his short time in Turin:
In recent seasons, the Portuguese has started campaigns slowly before finding his groove when it matters late on. Juventus will be hopeful their new no. 7 does begin clicking into gear when the big prizes are up for grabs.
Ronaldo's return to United would have given the club a lift, as he's a fan favourite and still one of the best players on the planet. But Mourinho has more pressing issues to resolve in his team than a lack of goals.