First, here is his start on 18 May, against the Mets:
Then, a start from exactly one year ago, 18 May 2013, against the Padres:
And, finally, a start at about the same time of year against the Mets, 4 June 2013:
The first thing to note is that Zimmermann is throwing a lot more change-ups, and a lot fewer breaking balls. Theoretically, this is good, because breaking balls apparently lead to arm trouble. However, his fast ball does not appear to be moving as well as it did in his starts last year. In fact, even his change-up isn't moving quite as well as it did last season. His PitchF/X velocities as recorded at Fangraphs.com show his average fast ball travelling at a slower speed, while his breaking balls are a bit faster. Going back to the charts note that the fast ball spread is not as tight this year as it was last. The slower ones are a bit slower, and there aren't quite as many faster ones. Now, the charts also show his numbers so far for 2014 are comparable to the 2012 ones rather than the 2013 ones, and that overall he has gained velocity since 2009. It's all a bit of a muddle, and I have no conclusion to offer, only that question. Should we worry? Let's create a watching brief on this one.
EDIT: I stupidly forgot to mention that Dave Jageler's comment about Zimmermann being 'wild in the strike zone' during the radio brodcast on 18 May against the Mets was what sent me to look at BrooksBaseball.com In fact, it seems more true of 2013 games than the one on 18 May.
*Jayson Werth is my favourite positional player, but I like pitchers best of all.