A couple days ago, Jared Diamond of the Wall Street Journal wrote about the Mets' forthcoming decision to play Juan Lagares or Eric Young Jr. on an everyday basis. In the piece, he noted the difference between Mets pitchers' ERA in games Lagares started in center field and those he didn't.
Specifically, the Mets had a 3.55 ERA when Lagares started in center and a 4.03 ERA when he did not. Of course, that didn't mean the difference was solely attributable to Lagares's defense. Our own Russ asked what the difference between the Mets' FIP with and without Lagares was, as "something somewhat of a control." That's the perfect way of putting it.
FIP, a fielding-independent statistic, measures pitcher performance only by walks (and hit batsmen), strikeouts, and home runs. It takes Lagares—and the rest of the Mets' defenders—out of the equation, rare home run robberies by outfielders aside. it gives us an idea of how well the pitchers themselves performed.
When Juan Lagares started in center field, Mets pitchers had a 3.70 FIP. When he didn't, Mets pitchers had a 3.90 FIP. In short, the pitchers pitched better when Lagares played than when he didn't. With Lagares in center, Mets pitchers struck out 7.4 and walked 2.6 hitters per nine innings. Without him in center, they struck out 7.3 and walked 3.0 hitters per nine. And they gave up 0.91 home runs per nine with him and 0.95 home runs without him. That's a partial explanation for the difference in ERA.
There's little doubt that Lagares was a good defender in center last year, but his defense was not the only factor in the team's improvement in run prevention after his arrival. Jon Niese and Dillon Gee struggled mightily early last year, but both pitchers turned their seasons around and finished strong. While Lagares debuted in late April, he started much more often beginning a couple months later. And, of course, Lucas Duda's tenure in left field mostly came to an end once Lagares began playing on a regular basis. That did not hurt the team's run prevention efforts.
Lagares's defense was likely a factor in the Mets' pitching staff posting a better ERA—lower than its FIP, too—when he was in center field. The difference just looks a bit less when taking all of the other factors into consideration.