There aren't too many one-name guys in sports, and Manny deserves that kind of cred. You know the drill - he can be scary, scary good, if only he can be motivated. You know, like in a pennant race. Or in a contract year. Or hitting in a tiny little ballpark that can inflate his numbers.
It's all right to be a bit concerned as Twins fans. When he joined the Dodgers two years ago with similar incentives, Manny hit about .400 with 17 HR in 187 AB. That's basically cross-pollinating Ted Williams with a roided up Mark McGwire. Which is scary as an opponent, and probably scarier as a media member.
Plus, he was clutch. In those two months with the Dodgers, he posted a 3.45 WPA. One way to think about that would be to say that he basically won 7 games all by himself over those two months. Another would be to compare it to the highest hitter's WPA on the Twins, which is Justin Morneau (still) with 2.69.
Think about that. Manny outdistanced Morneau by about 40% with 33% less time. He basically had twice the impact on a monthly basis - and Morneau was one of the leading candidates for MVP.
It's not silly to be worried about Manny's impact. Indeed, given his history, it would be silly to not be concerned.