Is There An Adjustment Time for Players Changing Leagues?

Last season, Prince Fielder and Albert Pujols moved from the National League to the tougher American League. A drop in production was expected… and occurred. As the season went on, they began to hit better. With a sample size of two, it seems to take players a while to adjust to a new league and its pitchers.

By looking at players who changed leagues, let’s try to determine if there is an adjustment period to the new pitchers and parks. These transitioning hitters could then be bought at a discount during their adjustment time.

Albert Pujols was the big free agent signing last off season by making the move from the Cardinals to the Angels. From the season’s start, he struggled and never got his first home run until May 6th (29th game). Over the season’s first half, he hit a meager .268/.334/.460. In the second half, he hit better with his triple slash line improving to .305/.354/.581.

Prince Fielder didn’t struggle to the level Pujols did, but he still did had a better 2nd half of the season. During the first half of the season, he hit a respectable .299/.380/.505. In the 2nd half, it jumped to .331/.448/.558.

Hitters could struggle as they get used to new parks and pitchers. Mitchel Lichtman determined hitters perform better the more they see a pitcher. A batter hits .006 wOBA higher after seeing a pitcher 13 or more times in season than when he sees him for the first time. While the difference is not huge, it is measurable.

To see if players initially struggle when they change leagues, I looked at every hitter who had 200 plate appearances in one league in a season and then saw how they did the next season in another league. In all, I looked at 153 matched seasons over the last five years.

While it was not the focus of this study, I had the numbers available and went ahead and looked at how hitters performed going from one league to another. The impact of changing leagues on position players has been studied previously, but it is important to get a baseline for the rest of the study.

All -0.001 -0.002 0.001 0.002 0.0012 0.0053 -0.0009 0.0008 0.63% -0.20%
NL to AL -0.004 -0.012 -0.005 -0.001 0.0009 0.0067 -0.0028 0.0007 0.99% -1.23%
AL to NL 0.003 0.008 0.007 0.005 0.0015 0.0038 0.0011 0.0008 0.24% 0.90%

As we probably expected, players perform worse in the American League than they do in the National League. In three instances, the numbers go in the same direction. Stolen base and home run numbers both improve, but the totals aren’t much. It works out to an increase in .7 SB and .5 HR in 600 at-bats. The change in strikeout rate is a little more important, but not too much. The change in strikeouts leads to four extra strikeouts over 600 PA.

Besides just looking at the actual increase, we can calculate the chances of improving or getting worse after the switch in leagues:

NL to AL
Stat Improves Worsens
AVG 42% 58%
OBP 37% 63%
SLG 47% 53%
ISO 53% 47%
K% 37% 63%
BB% 28% 72%
SB/PA 55% 45%
AL to NL
Stat Improves Worsens
AVG 48% 52%
OBP 57% 43%
SLG 55% 45%
ISO 57% 43%
K% 45% 55%
BB% 59% 41%
SB/PA 54% 42%

Almost everything seems in line with our expectations. Players perform worse when moving from the NL to the AL and better if going from the AL to the NL. One piece of information stil sticks out — strikeout rate. Hitters moving to the AL improve across the board, except with K%. I am not sure why. Does the player feel like he needs to show his new team he is more aggressive? Does it take longer to adjust to the movement and/or speed of strikes a pitcher throws instead of the pitches out of the strike zone (which you might find reflected in the walk rate)? Are hitters taking more pitches during the adjustment time and strikeouts are up? Not one answer seems 100% plausible, but the difference exists.

Jeff Keppinger is an example of a player who changed leagues between the 2011 and 2012 season. In 2011, he hit .277/.300/.377 with a 3% BB% and 6% K% in the National League. His 2012 numbers all improved except one: .325/.367/.439 with a 6% BB%… and a 7% K%. His improvement in that walk rate was the highest among all players moving from the NL to the AL. The increase was an anomaly because only about one in four players moving to the AL see an jump in their walk rate.

After running these numbers, I took a look at how each moving player performed in the season’s first half compared to the second half in their new league. Again, I speculated it took a few months for players to adjust to the new league’s pitchers and parks.

All -0.001 -0.005 0.001 0.002 -0.0023 -0.0092 0.0012 0.0010 1.3% -0.5%
NL to AL 0.000 -0.007 0.003 0.004 -0.0011 -0.0027 -0.0021 0.0012 1.5% -1.0%
AL to NL -0.003 -0.002 -0.002 0.001 -0.0038 -0.0166 0.0049 0.0009 1.0% 0.1%

Overall, the results are mixed. Batting average and on base percentage go down a tad while slugging and isolated power increase. Very negligible changes overall. One note, the K% increase in the second half for both leagues surprises me a bit. It seems hitters should improve as they become more familiar with a pitcher and the K% should then go down. This change may explain the why the K% increases for both leagues, but what happens from the first half of the season to make a player strikeout more? I just don’t have a good explanation.

None of the changes are even close to those seen by Fielder and Pujols last season. For every increase in production from the first half of the season from a player, there is another example of a player declining. For example, here are the OPS values (used for simplicity) of some hitters who struggled, instead of improving, during the second half of the season in a new league.

Name Season 1H OPS 2H OPS
Jason Bay 2010 0.779 0.510
Adam Dunn 2011 0.597 0.519
Melky Cabrera 2012 0.803 0.606
Colby Rasmus 2012 0.821 0.515

Going into the 2013 season, several big named players have changed leagues: B.J. Upton, Melky Cabrera, and Shane Victorino. Each may struggle or thrive in their new settings. Also, as the season goes on, their output may change. The change shouldn’t be attributed to adjusting to a new league. Instead, fantasy owners may need to look for other causes for the players’ ups and downs like injuries, new swing mechanics and good old luck regression.

Last season, the two big offseason free agent signings performed better in the second half of the season. By looking at all players changing leagues over the past five years, though, players don’t perform better (or worse) over the first part of the season compared to the second half. A fantasy owner should not buy low on a player struggling over the first half of a season in a new league expecting a rebound. The player could get better like Albert Pujols did in 2012 or decline even more like Adam Dunn did in 2011.

Jeff, one of the authors of the fantasy baseball guide,The Process, writes for RotoGraphs, The Hardball Times, Rotowire, Baseball America, and BaseballHQ. He has been nominated for two SABR Analytics Research Award for Contemporary Analysis and won it in 2013 in tandem with Bill Petti. He has won four FSWA Awards including on for his Mining the News series. He's won Tout Wars three times, LABR once, and got his first NFBC Main Event win in 2021. Follow him on Twitter @jeffwzimmerman.

2 Responses to “Is There An Adjustment Time for Players Changing Leagues?”

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  1. ror0071619 says:
    FanGraphs Supporting Member

    Just curious. Does Unintentional Walk Rate change as much when switching leagues. Just thinking number 8 hitters switching to the NL (while probably not a huge sample size) may be intentionally walked more often.

  2. Jeff Zimmerman says:
    FanGraphs Supporting Member

    ror-I didn’t run the numbers. Let me see what I can did out, but no promises.