Luis Antonio Valencia, a £16 million signing from Wigan Athletic, was always likely to end up at Old Trafford this summer.
Ferguson has been watching the Ecuadorian for more than a year, and his mind was almost certainly made up when Valencia gave John O'Shea a torturous evening during United's 1-0 win over Steve Bruce's side at Old Trafford in January.
Had Ronaldo stayed, that may have been it. In the midst of the bitter, and faintly embarrassing, war of words between Ferguson and Rafa Benitez, the Scot admitted he did not feel his side needed a huge amount of strengthening this summer.
Ronaldo's departure, confirmed in a terse statement on United's website today, has changed all that. Suddenly the flaws are obvious.
Valencia solves the most apparent, the need for another wide man, the Portuguese's nominal position. But Ronaldo, obviously, was so much more. Not only was he goals and assists, and free kicks and penalties, he was target man and No 10. More importantly, he was a mask for all of United's ills.
Ferguson would probably admit that his side, however feted until that 2-0 humiliation at the hands of Barcelona in Rome, was far from perfect. The central midfield, certainly, is weaker than either Chelsea's or Liverpool's, thanks to the uncertainty over whether Owen Hargreaves will ever play again.
United's famous strength in depth weakens a little with every year that passes on to the legs of Paul Scholes, Ryan Giggs and Gary Neville. Park Ji-Sung, Nani and Zoran Tosic are hardly wingers to terrify defenders.
Dimitar Berbatov has not settled well in his first season, often a peripheral figure, rarely employed for the key games.
Carlos Tevez may be an idol for the fans, but Ferguson clearly does not believe he has the quality required to be a regular, effective presence at the highest level.
In goal, Edwin Van der Sar is showing signs of wear and tear, and Ben Foster and Tomasz Kuszczak are yet to prove they can replace him.
Add to that the loss of Ronaldo, the team's standard-bearer, the man who could win games when all else failed, and Ferguson has much to think about.
First and foremost, he must replace Ronaldo's goals, and then look to strengthen elsewhere. £80 million might be burning a hole in his pocket, but his need now is more obvious, the salve of Ronaldo absent, the spectre of Madrid looming.
Reports in Spain suggest he has already lost out to Real on Karim Benzema, the Scot's undoubted first choice.
Should Franck Ribery leave Bayern Munich, it seems it will be for the Bernabeu, and Kaka has already gone. Samuel Eto'o, David Villa and Zlatan Ibrahimovic all seem beyond United's reach.
Where he will find the goals is unclear, because it certainly won't be from Valencia, who has managed just seven in three years in the Premier League.
In midfield, too, there are tough decisions to be made. Valencia's David Silva could provide more balance and class on the left - at a reasonable price - but there is still a hole in the middle, unless Hargreaves recovers from his chronic knee troubles.
Van der Sar is likely to get a stay of execution, purely because Ferguson has greater troubles to ponder.
Of course, these are no doubt problems most other fans would love to have. With no more signings this summer, United would almost certainly finish in the top two next season. Their defence is solid, their squad full of class. But Ferguson has set himself high standards.
At Old Trafford, nothing but the best is good enough, and even £80 million cannot hide the fact they've just sold their best to Spain.