But the key thing to notice is that Johnson’ ERA in 2006 was not an accurate reflection of his pitching ability; his peripherals translate to an ERA much closer to 4.00 than the 5.00 he posted. The reason for the disparity is that Johnson pitched very poorly with runners on base, yielding an ugly .321/.363/.564 line as compared to .206/.271/.324 with the bases empty.
Now, those sorts of splits aren’t always about luck. There could be something about pitching out of the stretch that is driving that discrepancy, perhaps something related to Johnson’s balky back or his heavy use of his slider. But let’s take a deeper look at the numbers:
Johnson struck out 20.6% of batters with the bases empty, and 19.8% with someone on base. Not much difference there.
He yielded an unintentional walk to 7.1% of batters with the bases empty, and 9.9% with the bases occupied. That difference is a little more substantial, but it’s also a natural adaptation that a lot of veteran pitchers like Tom Glavine make: there’s more incentive to challenge a hitter when there’s nobody on base, leading to comparatively lower walk rates.
Johnson’s BABIP with the bases empty was .239. With runners on, it was .369. This is almost certainly a matter of luck.
So perhaps the big difference is that Johnson was getting hit up for extra bases more often with runners on base? Well, this is true: his home run rate was nearly twice as high with runners on (4.6%) than with the bases empty (2.5%). But the funny thing is that Johnson was actually doing a better job of keeping the ball on the ground with runners on. His groundout-to-airout ratio was 1.09 in those situatuons, as opposed to 0.78 with the bases empty. In other words, more bad luck.
I’ve got news for you: the Yankees might not be trading their #4; they might be trading their #1. Johnson’s ERA PECOTAs out at 3.52, which is the best in the Yankee rotation by some margin. You can take that PECOTA with a certain grain of salt because it’s so hard to find appropriate comparables for Johnson. But the names that PECOTA does come up with — Roger Clemens foremost among them — are a reminder that you shouldn’t bet against a great pitcher until you absolutely have to.