Fantasy Baseball: 3 Things We Learned in Week 4

Baseball fans love their stats. We devour them, dissect them, and build our fantasy rosters around them. Each week of the 2021 baseball season, we will be gifted with another statistical sample size of pitches, plate appearances, and playing time. Knowing it often takes hundreds or even thousands of pitches or batted-ball events for trends to normalize, how should fantasy managers adjust to the ebbs and flows of weekly player performance?

Each week during this season, this piece will look at trends that have emerged over the past week and determine if it is signal or noise moving forward. What is prescriptive in helping build winning fantasy teams and what can be ignored as small sample size noise? Hopefully, we can make sense of what has just happened to help us make smarter roster and free agent budget decisions.

Let's take a look at some of the data from the fourth scoring period of the fantasy baseball season.

Where Have All The Fastballs Gone?

It's somewhat of an exaggeration to say that pitchers are relying less on their fastball and starting to move away from the pitch. According to FanGraphs, fastballs are still thrown 50.9% of the time in 2021, far and away the most of any pitch in the league. But when you start looking at trends of pitch mixes league-wide, you can see that something unique is happening to pitcher arsenals across Major League Baseball.

If we go back to just 2019, fastballs were thrown 52.5% of the time. That number jumps to 55.6% in 2017 and 57.7% in 2015. So what are pitchers throwing instead? Here is a look at fastball versus slider usage every year since 2015, from FanGraphs data through Sunday's games:

As you can see, there's a drop in fastballs thrown each year and a rise in sliders thrown each year. At this rate, we will dig into the data in the 2022 season and see that fastballs are no longer a majority of pitches thrown across MLB.

This answers the what, but what about the why? Why are pitchers moving away from the fastball and trending more towards breaking pitches, and how can it impact our fantasy rosters?

On the mechanical side, recent research has shown that pitchers who have undergone Tommy John surgery are actually more correlated to pitchers who throw more fastballs than those who throw more breaking pitches, contrary to popular belief. The more velocity a pitch has also puts more torque on an elbow compared to slower breaking pitches, which can eventually lead to the elbow breaking down.

For fantasy purposes, Clayton Kershaw can serve as a mini-case study for how this can be helpful. He has undergone a mini-resurgence this year after posting the highest barrel rate of his career last year and posting the first ERA above 3.00 as a professional in 2019. But he has his lowest fielding independent pitching (FIP) since 2016 this season (2.75) and his lowest expected earned run average (xERA) since 2017.

How can this be when his fastball this year (91.4 miles per hour) is sitting two MPH lower than his career average (93.4)? The answer is he is only throwing his fastball 35.7% of the time in 2021 (56.5% for his career) but is throwing his slider 44.7% of the time (25.8% career). That slider percentage is first among all starters with at least three starts this year, while his fastball percentage is 159th. According to FanGraphs, Kershaw's slider is the 19th best in the Majors this year, while his fastball only ranks 40th.

If you have a pitch that performs better than your others, why not throw it more often? Perhaps we can use these pitch mix trends to unlock potential or undervalued players throughout the season. Pitchers who trend towards throwing their more valuable pitches more often, no matter what those pitches are, could be great buy-low candidates as the season moves along.

Don't Fear the Slow Start for Dominic Smith

The new-look New York Mets have certainly had a strange start to begin the year. The Mets have played only 17 games when the rest of the teams in the National League have played between 19-22 games. Starting the year with COVID postponements, and then enduring two weather cancellations plus one game that was suspended after one batter, it's no wonder a player like Dominic Smith is struggling to get the bat going to its full potential through four weeks.

But don't lose hope because Smith is going to start smashing soon.

On the surface, a .232 batting average, two home runs, and a .629 OPS through 17 games don't scream that Smith is on the verge of a breakout, but looking under the hood gives a much clearer picture that the engine is humming and is about ready to take off.

It can be dangerous to look at "expected" stats this early in the year, as they can tell a misleading story with so few plate appearances for batters, but when you see such drastic splits -- as in the case of Smith -- we certainly need to sit up and take notice.

Despite a .232 average, Smith's expected batting average (xBA) is .336, the 17th best in the league. Similarly, his slugging percentage is a putrid .375, but his xSLG is .656, 20th in the league. Again, the difference between his weighted on-base average (wOBA - .271) and expected weighted on-base average (xwOBA - .407) is astonishing.

The differences between those three sets of numbers put Smith in the top-12 largest margin of all three statistics, suggesting some positive regression is coming. But what's causing the major difference between expected and actual outcomes?

The best I can tell, according to this chart from Baseball Savant, is that Smith is making much less contact on pitches in the zone than he has during the rest of his career. He makes contact with 80% of pitches in the zone for his career but is only at 72.8% in the first few weeks of this season. That is likely just a timing issue (related to the stops and starts mentioned above) instead of a physical one or an eye problem, so expect that percentage to shoot up, leading to more of the valuable fantasy numbers we are looking for.

Add or Trade For Michael Kopech

Michael Kopech (57% rostered in Yahoo leagues) - If you watched any of Michael Kopech's start against the Texas Rangers on Sunday, you probably realized you were witnessing something special. Originally slated to start the year in the bullpen, Kopech has been working to stretch out his appearances, finally culminating in a 5-inning, 10-strikeout effort against the Rangers yesterday.

Originally drafted in the first round of the 2014 draft right out of high school, the hill Kopech had to climb to get a starting gig has been a steep one. In fact, his time in the Majors this year is just his second stint on the big club, the first coming back in an audition in 2018 when he started four games when he had an 86 ERA+ (100 is average), a 1.53 WHIP, and 6.16 FIP (which is scaled to be similar to ERA).

Kopech decided not to participate in the 2020 season, so pundits all wondered how far behind he would be entering 2021 and how long it would take him to hopefully display some of his dominant arsenal that has been demolishing batters through the minor leagues.

It appears all he needed was four weeks.

There is simply no way the White Sox can send Kopech back to the pen for two-inning stints now, right? According to Baseball Savant, Kopech's xERA is 1.37, the third-best among all starters and behind only Jacob deGrom and Corbin Burnes. Maybe you have heard how well they are doing this year? Similarly, his xwOBA is also third at a microscopic .193.

Living up to its minor league hype, Kopech's velocity on his pitches so far is what has set him apart. He already possesses the 13th-fastest four-seam fastball (96.3 mph) and ninth-fastest changeup (89.9 mph) in the Majors this year. For context, Kopech's changeup is the same velocity as Hyun-jin Ryu's fastball.

In fact, Kopech's fastball has been so dominant this year, that if he gets you to a two-strike count, your at-bat is basically over. His four-seamer ranks sixth in all of baseball in put away percentage at 36.7%. What that means is that with two strikes, Kopech is striking opponents out 37% of the time with his fastball on the very next pitch. That's better than Tyler Glasnow, Max Scherzer, Gerrit Cole, and a number of other elite starters.

As he develops the ability to go pitch longer into games, we should continue to see his elite stuff become more effective, so if he is available in your league, now is the time to pick him up.