Lee is a bona fide ace, the type of pitcher who has made the difference in years the Bronx Bombers were unsuccessful in the World Series.
In 2001, the Arizona Diamondbacks negated three nights of Yankee magic. They unleashed the unhittable pitching of Randy Johnson and New York lost the series in seven games. Yankee executives became so obsessed with Johnson they signed him a couple of years later-- when he was too old to be effective.
In 2003, the Yankees were the better team, but the Marlins had one ace pitcher who couldn't be stopped. In fact, Josh Beckett's complete-game domination of the Bombers in Game 6 is the closest comparison to Cliff Lee's opening night act.
Now, of course, all is not lost. The Braves of '96 and '99 had aces (see also Maddux, Greg and Smoltz, John) but the Yankees overcame them. Indeed, in '96, the Yankees lost the first two games of the World Series, at home, but rallied to win the Series anyway. (Fans will recall the key hit in Game 6 was delivered by current manager Joe Girardi.)
But this time around, New York will have to account for Cliff Lee. He's going to pitch again at least once, maybe twice, and that leaves the Yankees two options. They can beat Lee the next time they face him. Or, they can win all the other games in the series