The White Sox and Matt Thornton agreed to a two year-contract extension with a third-year option Sunday, keeping Thornton under team control through 2014.
Over the last three seasons, Thornton has been worth 6.6 fWAR—the highest total for any reliever not named Mariano Rivera. Or, to look at it another way, Thornton has been the best left-handed reliever in baseball over the last three seasons.
That shouldn't come as a surprise for White Sox fans who have watched Thornton mow down batters to the tune of an 11.09 K/9, the fourth-best strikeout rate (and best among left-handers) among relievers in the last three seasons. But how long can opponents expect Thornton continue his reign of terror?
Thornton will turn 35 in September, and if the White Sox pick up his option for 2014, he'll turn 39 in the final weeks of his contract. That's awfully old for any player, but it wouldn't be unprecedented for Thornton to continue his success into his late 30's.
Switching to Baseball-Reference's WAR, there have been 60 instances of a reliever between the ages of 35 and 39 posting a WAR of 2.0 or higher since 1970. Not surprisingly, quite a few of those instances have been in the last decade.
So just because Thornton will be in his late 30's for the duration of this contract doesn't mean he should be expected to see a serious decline. He'll probably see some decline, but any significant decline will coincide with a significant drop in velocity. Thornton hasn't seen that yet.
Maybe it would've been more prudent for the White Sox to try to extend Thornton after the 2011 season, but if the team tried to do that, they'd risk losing him to free agency. So on one hand, the Sox could make sure Thornton's velocity doesn't start to dip; but on the other, if it didn't they may lose him.
The money isn't an issue—$5.5 million per year gives the Sox good surplus value assuming Thornton's performance doesn't take a major step back. In 2010, 1.4 was worth $5.5 million, and that dollar value will likely increase over the next few years. So as long as Thornton doesn't regress below, say, 1.0 WAR, this can hardly be viewed as a bad deal.
But the risk is that Thornton will regress with age—or get hurt—and not be worth that money. It's not an incredibly large risk, but the risk certainly exists and is greater than if Thornton were five years younger.
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