What fun is that? You know you're just jealous of sportsnyan's awesome poptar cat avatar 
But I didn't single out ERA, or any other single factor. I said "by almost any measure". If one of those measures "regresses" and his ERA suffers as a result, so what? I thought ERA was mostly meaningless to you anyway? 
C'mon Chief. Of course ERA isn't meaningless. The whole point of stats like FIP and SIERA is to give us a better idea of (1) how much the pitcher actually contributed to the team's chance of winning, and (2) how much he is likely to contribute going forward. The larger the number of innings, the better ERA is at telling us both things. RA/9 is probably even better, but it's not widely available and rarely differs too much from ERA. However, in Zimmermann's case, he hasn't pitched enough for ERA to be very useful as a predictor. SIERA is a much better predictor of future ERA given a smallish sample size like Zimm's than ERA is, which is why I care about it, but if it didn't correlate so well with future ERA I wouldn't give a crap about it.
But if you said "by almost any measure" he's a top twenty pitcher, you're wrong. Zimm's elite in categories related to HR and BB rates, which is obviously a good thing, but that's about it. He's got below average strikeout rates, has a GB/FB below 1, has an (identical) xFIP and SIERA that aren't top 20, his LOB% is below 70%, he has a negative clutch rating... and, if you want to go really old-school, he has a losing record. So I'm going to guess that you're calling him top twenty because of his ERA--his low walk rate (5th in the majors among qualified starters) and HR/FB rate (2nd) are much too good for you to just have called him "top twenty." His potentially regressing in one of those categories isn't a "so what" thing, it's a
huge deal because a lot of his value is tied up in his rarely giving up dingers or free passes.
