In the 2010 World Cup, Greece faced Argentina, and Sokratis Papastathopoulos—now of Borussia Dortmund—was set the seemingly unenviable task of man-marking Lionel Messi.
He did an extremely effective job, and given Greece's group-stage opponents, it could be worth Santos asking the defender to dust off his man-marking skills on Ronaldo.
Against Cote D'Ivoire, if Yaya Toure plays at the tip of a diamond, Papastathopoulos could again help his midfield colleagues keep the Manchester City man quiet.
Against Japan, he could do the same to Shinji Kagawa. Against Colombia, he could be asked to do a job on either Jackson Martinez or Falcao, depending on the latter's fitness.
If Ronaldo plays wide for Portugal, this is probably not worth pursuing. If he plays centrally, it may be. Given what an excellent job Papastathopoulos did against Messi, it is surely worth giving him the chance to complete the set.