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Real Madrid is loaded with potential; players with great crossing ability and players with great shooting.
Madrid also have some dribblers, but they don't have enough game-changers.
Ronaldo is obviously the best game-changer in world football. He can decide the outcome of a match often all on his own—and often—that is the case.
Still, Madrid have no one that can be the x-factor. Adding Neymar would allow Madrid to utilize their defensive midfielders more effectively, and allow more freedom and creativity to flow from the attack.
No one is as skilled with their feet or as unpredictable as Ronaldo. Consequently, this makes marking Ronaldo easier...and thereby, shutting down Madrid easier.
Neymar would relieve pressure off Ronaldo and free up the Portuguese forward to attack more space.
Defenders would have to pay just as close attention to Neymar as they would Ronaldo. For instance, look at Barcelona.
Iniesta, Pedro, Messi and Sanchez are all great dribblers with pace. These players attack the space and open up pockets for their teamates to attack and exploit.
With Neymar pulling defenders out, Ozil, Alonso and Co. can adequately attack through the middle.