I think McCourty himself in an interview said that his retirement gives the Patriots a chance to be more unpredictable in their backend coverages. He said that teams knew where he was going to be 99% of the time, but now any of the safeties could be deep, or could be low, or could start at one then rotate to the other. Now, will that unpredictability be enough to make up for his generally superior deep range (and his ability to communicate the playcalls to everyone in real time), we'll see. But I doubt they're looking for anyone to really replace McCourty's role. They'll tweak the defense to take advantage of the players they have.