What Happened to Eddie Rosario’s Defense?

I will start this article by saying, I am only referencing fielding metrics generated by Statcast that are available on Baseball Savant. While many of the defensive metrics on sites like Fangraphs and Baseball Reference could be helpful, they are, at least for outfielders, outdated, and nowhere near as accurate for measuring an outfielder’s ability to track down flyballs as Statcast is. The reason for this is that Statcast actually tracks player and ball movements to quantify the time and distance that an outfielder has to track down a flyball, and calculates catch expectancy from that. Other metrics, like UZR and DRS, rely on the eye test from humans, which is a much less precise process. If you would like to read more about how the Statcast fielding metrics are calculated, you can do so here.

That being said, let’s start diving into the numbers. We will start by looking at Eddie Rosario’s expected catch percentage vs. his actual catch percentage. So far this year Rosario has an expected catch percentage of 86 percent. That is to say, an average outfielder would catch 86 percent of the fly balls hit to Eddie Rosario this year. However, Rosario’s actual catch percentage stands at just 81 percent, giving Rosario a catch percent added of -5 percent. While this might not sound like a lot, it actually is. Among the 97 outfielders who have fielded at least 50 fly balls this year, only four outfielders have an expected catch percent added worse than Rosario's. This is a severe drop off from the +2 percent, -1 percent and -1 percent, that Rosario averaged in 2016, 2017 and 2018 respectively.

Using these metrics, we can calculate Eddie Rosario’s outs above average. How this metric works is, if you have a fly ball hit to you that has a 75 percent catch probability, and you catch it, you get credit for 0.25 outs above average on that opportunity. However, if you fail to catch it, you get credit for -0.75 outs above average on that opportunity. In 2019, Rosario’s outs above average currently stands at -9 outs, which is on pace for 13.5 outs below average. For comparison, Byron Buxton currently has an outs above average of 12, which means Rosario has negated 75 percent of the value Buxton brings in the field defensively with his play in the field.

So, what has caused Eddie Rosario to experience such a drastic drop off, from roughly league average outfielder, to one of the worst defensive outfielders in baseball? For that, we will need to dive even deeper. We will start by looking at Rosario’s sprint speed, which is another metric tracked by Statcast. This year, Rosario’s average sprint speed is 27.2 feet per second, which is roughly league average. However, from 2016 through 2018, Rosario averaged 28.0 feet per second. Again, for comparison, Byron Buxton’s sprint speed this year is 30.3 feet per second, so a 0.8 feet per second drop is a significant decline.

However, there is more to being a good outfielder than just being able to run fast, though that does help a lot. Getting a good jump on the ball, getting up to speed quickly, and taking a good route to the ball are all important factors in being good at tracking down fly balls. Luckily, thanks to the player movement tracking technology, via Statcast, we can measure each of those factors, using some newly introduced metrics referred to as Reaction, Burst, and Route.

We will start by looking at reaction, which measure the amount of ground an outfielder covers in the first 1.5 seconds after the pitch is thrown. From 2016 through 2018, Eddie Rosario measured in at 0.3 feet above the MLB average in reaction. However, in 2019 Rosario is at -0.8 feet below average in that initial reaction stage. The next metric we will look at is burst. Burst measures the feet that an outfielder covers from the 1.6 second mark, to the 3 second mark of the play. In other words, how fast the outfielder is accelerating. Again, looking back from 2016 through 2018, Rosario averaged 0.0 feet above average in burst, or exactly league average. That number has fallen all the way down to 1.2 feet below average in 2019. The final metric that we will look at is route, which measures the outfielder’s route efficiency, and how many extra feet they are needing to run by taking a poor route. From 2016 through 2018, Rosario gained 0.2 feet, per opportunity, on the average MLB outfielder due to taking good routes. In 2019, Rosario is gaining 0.3 feet per route above the average MLB outfielder.

It is apparent that Rosario has taken a considerable step backward in not just sprint speed, but in his all-around athletic skills as well. So, the question is, how much has this hurt Rosario? The table below breaks down how many feet Rosario is losing, on a per fly ball basis, from where he was in the three previous seasons.

The first thing to note from this table is the Feet Gained for the Feet Per Second category is multiplied by two, and there is a reason for this. For starters, the average fly ball is in the air for just shy of five seconds (4.97 seconds, to be exact), hence, the feet per second drop off is amplified the longer the ball is in the air. However, feet per second is measured while a player is at max speed, not while he is accelerating. Since it typically takes about three seconds between the release of the pitch, and the moment the fielder reaches max speed, the feet per second is not factored in until after that point. Therefore, the average fly ball hang time of five seconds, minus the three seconds it takes to reach max speed, gives us an average of two seconds at max speed per fly ball.

After taking that into consideration, we can calculate the rest of the factors, and see that Eddie Rosario’s ability to track down a fly ball has gone down by an average of four feet per fly ball. While on most fly balls, four feet won’t make much of a difference, over the course of a season, the number of fly balls where the extra four feet would make the difference starts to add up.

Knowing this, the Twins need to ask themselves, is this something they can work with Eddie Rosario on? Has he simply lost a step athletically that he might not recover, or is Rosario simply having an off season in the field? Whatever the answer is, it is important for the Twins to work with Rosario, so they can either help him improve, or start to factor in this decline for their future evaluations of Rosario as an all-around player.