(Tommy Gilligan-USA TODAY Sports)

Stephen Strasburg has struggled of late. He has struggled in the early innings of games. He has struggled with his fastball. And he has struggled to keep himself known as the ace of the staff.

Going by conventional stats I can see why people would start to believe Jordan Zimmermann might be the Nationals’ best pitcher instead of Strasburg. Three of the four pitchers not named Strasburg have a better win percentage and four have a better ERA. Plus, Strasburg has yet to pitch either a shutout or a complete game.

Zimmermann, on the other hand, has pitched solidly throughout the season. He has a 5-4 record to go along with a 2.95 ERA (26th among starters qualifying for the ERA title) and has two complete games plus a shutout. Even advanced metrics such as wins above replacement put Zimmermann only slightly behind Strasburg this season.

And the more Strasburg struggles, the more Zimmermann looks like the ace to some, if not most, of Nats fans. But make no mistake, Strasburg  is the top pitcher on this staff.

Let’s start with defining what exactly it means to be an ace of the rotation: they need to strike out batters frequently, provide quality starts and not be responsible for many runs on the board.

Strasburg has struck out 123 batters and walked 23 in 104.2 innings. That’s almost 40 more strikeouts than Zimmermann.

And even with his struggles of late, no one induces more swinging strikes than Strasburg.

Now let’s take a closer at Strasburg’s inflated ERA, which is a poor estimate of a pitcher’s true talent level.

ERA is not a good predictor of future success, as Earned Runs are dependent on multiple factors outside of the pitchers control: defense, umpiring, the judgment of a scorekeeper, etc. Pitchers aren’t held accountable for runs scored as a result of an Error by one of their fielders, but they are held accountable for runs scored on bloop hits that get by slow or poor defenders. If a pitcher has a poor defense behind him, he will likely end up with a higher ERA than he should have.

Instead we can look at Fielding Independent Pitching (FIP), which measures what a pitcher’s ERA should have looked like by factoring in results a pitcher can control: strikeouts, walks, hit by pitches, and home runs. Lower is better, and a FIP of 2.90 or below is considered “excellent” while the league average gravitates around 4.00. And we can see that Strasburg once again rises to the top.

Using Expected Fielding Independent Pitching (xFIP), which replaces a pitcher’s home run total with an estimate of how many home runs they should have allowed, makes Strasburg look even better.

Strasburg has also given the Nats more quality starts — starts where the pitcher pitched at least six innings and allowed three or fewer earned runs – but has not benefited from much run support this season.

Strasburg may not be mentioned in the same sentence as Babe Ruth, but he is the ace of this staff.

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