Whether it's fatigue, rhythm, weather or another external factor — some pitchers get stronger as the season progresses, while others regress. The temperature and density of the air can have a substantial impact on breaking balls in particular. Also, some pitchers who rely more on feel can have varying results based on the humidity level.
There also could just be randomness or luck in some of the results. I looked only at pitchers with a minimum of 200 career innings — both before and after the All-Star break — but there is certainly still noise.
Let's take a look at 10 to 12 pitchers from each side of the fence. Don't forget that this is just one data point. In some cases, it can lead to value — while it doesn't paint the whole picture for others.
Second-Half Struggles
Not all second-half struggles are created equal. I'm not concerned with Bauer, who has transformed himself with a much better repertoire over the past year. He struggled after the All-Star break for a number of seasons, but had a dominant second half in 2017.
I personally monitor pitchers who've consistently performed worse in the second half over a significant sample size — and/or I can come up with a reason why. Guys such as Cueto, Zimmermann, Leake and Vargas fit the mold.
A 2018 All-Star worth keeping tabs on who just missed the innings threshold is Mike Foltynewicz. The Brave has a career 20-15 record with a 3.65 ERA before the break — compared to a 10-15 record with a 5.61 ERA after. Folty has really struggled in the second half in each of the past four seasons — especially last year when he finished 3-8 with a 6.34 ERA.
I'm also fascinated to see how Sale's arm holds up down the stretch. With the Red Sox a virtual lock for the postseason, they'll need their ace to get through a brutal AL. In 53 career games (32 starts) in September/October, the Boston lefty is 11-16 with a 3.78 ERA. That doesn't seem high until you realize Sale owns a career 2.76 ERA in all other months combined.
Second-Half Success
Cubs fans should be optimistic about Hendricks — starting tonight in St. Louis. He's been absolutely filthy in the second half over the past two years, going 12-4 with a 1.91 ERA in 27 starts.
As well as Clayton Kershaw has pitched throughout his career, he's been a 0.5 run better after the break. The southpaw has silly second-half splits.
You should also notice a number of Jays and Orioles on the list. That may set up for some nice fades of the Red Sox and Yankees — both of whom should be extremely overvalued the rest of the way. Get ready to hold your nose!
Speaking of the Orioles, Gausman is the type of pitcher I like to target after the break, as you can see his improvement on a month-by-month basis. Just take a look at his monthly career splits:
