The case for acquiring Justin Verlander basically came down to two key points:
a) His track record speaks for itself.
b) Going to a team with a better-than-non-existent chance to win a World Series would amp him up and he would recapture some form of magic that 34-year old pitchers rarely manage to recapture.
Turns out that, so far, both of those points have proven true. He's out-Johnsoning Randy Johnson. Though, to be fair, (and, like Randy Johnson) the Astros didn't acquire Justin Verlander for the regular season.
I don't know how you quantify the fabled Big Game pitcher.