Originally Posted by wolfboy
I don't know if I agree. Certainly, someone with a K/9 over 7 is much more likely to be an ace than someone below that threshold, but guys like Steve Carlton, Juan Marichal, Jim Palmer and Don Sutton all reached the Hall of Fame, and arguably "ace" status, while rarely having a K/9 over 7. More recently, Tom Glavine is a pitcher that wasn't close to a K/9 of over 7 for his career, but still was an arguable ace for much of it.
I'd also add that I don't think Cueto is an "ace."
In the modern game, the ability to miss bats is pretty important. While there are exceptions, it is rare for guys with K/9 under 7 to sustain success, and therefore reach "ace" status. Even guys like Roy Halladay and Chris Carpenter achieved new levels of success after figuring out how to strike out batters at a higher rate. Cueto's K/9 is headed in the wrong direction.