There are two questions here: Will he return to Old Trafford, and how would he fit into the team if he came?
Can Manchester United Re-sign Ronaldo?
Sir Alex was able to persuade him to stay just one more year before fulfilling his boyhood dream of joining Los Blancos.
There is clearly still a strong bond between them. Indeed, Ronaldo has described Sir Alex as a second father.
It was also clear that the player was deeply moved by the reception he received when he returned to Old Trafford with Real Madrid.
No opposing player has ever received such warm acclaim from the generous Old Trafford crowd. Not even the "other" Ronaldo.
Cristiano's current contract runs until 2015, but he has already indicated that he may not renew it. If he "sees it out," he could leave for free.
Real Madrid could not afford for that to happen. There may be a price and a time at which United could match their demands.
That leaves the Real Madrid Board with a dilemma. While they are one of the richest football clubs in the world, Financial Fair Play is here to stay. There is an uneven distribution of TV rights in Spain and the "Big Two" may need to let go of some of their income to other clubs.
It seems certain that Jose Mourinho will leave in the summer and the club has already been linked with a big-money bid for Sergio Aguero. Any new manager will want some input into his own signings in order to reshape the team.
At present Real Madrid play pretty much to Ronaldo's strengths, as Barcelona do to Messi's. Unlike the latter, however, it could be contended that Madrid rely too much on Ronaldo and may not have an effective "Plan B" if he is not in the team.
It has been noticeable recently how Barca have implemented a "Plan B" with David Villa in an orthodox No. 9 role.
So if a new Madrid manager decides to reshape the team to reduce the dependence on Ronaldo, what is to prevent him leaving altogether?
What Would Ronaldo Bring to Manchester United?
It is hard to categorise him because of his all-round attacking capability. If he was in the United side you would expect him to take most free kicks within 35 yards or so of the goal. Even if he doesn't score, the power of his dead-ball kicks disrupts any defence.
While he can play on either wing, he is not just a winger. There is an argument to be made for him being played as an out-and-out striker. Sir Alex tried this in his last match for United against Barcelona, but it didn't work because all the action was at the other end.
His most dangerous position is undoubtedly wide right or left, from which he can start his marauding runs across and into the box.
If United have a corner you would not expect him to take it on either side because he is so dominant in the air, as his goal in Madrid showed against United.
In short, the best way to use him is to let him loiter on either flank and go wherever he chooses. This means the rest of the team have to be able to fit in with this modus operandi.
Funnily enough, that is exactly the style Sir Alex has been evolving with the current United squad: a dynamic interchange of the front four or five attacking players.
How Would He Fit and What Changes would United Need to Accommodate Him?
The first thing you have to accept is that Ronaldo is not defensively adept. While his pace means that he can be a nuisance to the attacking team if he tracks back, you still want him to be available for a counterattack.
So his best defensive qualities will be seen in the penalty box on set pieces, where the power of his heading can be applied.
However, again the case can be argued that he should be the lone player left upfield ready for counterattack because of his pace, dribbling ability and all-round skill.
The biggest challenge would come if he started on the left flank, because Patrice Evra is not the best defensively and United could be vulnerable if they played together.
That means either Ronaldo must start on the right or you need a new or different left back. Alexander Buttner has better defensive qualities than Evra and is more disciplined if Ronaldo starts on the left.
Finally, to make the best use of him you need players who are capable of hitting him with a direct pass or able to drop the ball into space where he can use his sprinter-standard speed.
He has the skill to play in a dynamic attacking four or five;
He has the pace to be a major counter-attacking outlet;
He has the heading ability to be a huge threat at set-pieces;
He is comfortable on either flank and anywhere across the attack;
He does not have great defending skills.
Moulding a Team from United's Current Personnel
Pulling all these factors together, much of the team chooses itself.
Assuming Ronaldo sets up initially on the right of midfield, you need a right back who can defend as well as attack.
This means that Evra can play left back. In central defence Nemanja Vidic should be the "rock." His partner should be someone who can pass well out of defence. The choice is between Rio Ferdinand and Jonny Evans.
If Ronaldo is going to have pretty much a free role then you must have a holding midfielder who can also hit Ronaldo or space with a pass. Michael Carrick is the obvious choice.
Attack is the best form of defence, but the other attackers should ideally have some tackling capability.
The formation is also key. Ronaldo doesn't track or tackle back much, so a diamond would only work if he was playing as one of the front two.
So we should go with a modified 4-2-3-1, which can become a 4-1-3-2 when United are rampant.
Players like Ronaldo, Lionel Messi and to a lesser extent Gareth Bale scare defences stiff and occupy midfielders. At their very best United are an out-and-out attacking team who score more than they concede.
So we have gone for a very attacking team where we are relying on the defending capabilities of some attacking players to augment a strong central defence and a holding player.
That means Ashley Young must play on the left and a place has to be found for Wayne Rooney.
The Selected Team
So how would you accommodate Ronaldo in your team?