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In today’s society, the jack of all trades is endangered, if not extinct. In today’s workplace-and in sports-specialization is prized more than versatility. There are no more Chuck Bednarik types playing both ways, your mechanic probably can’t work on both GM and VW when it comes to cars, and lefty relievers can pitch until they’re ready to collect Social Security. Despite the obvious value of “roster expanders”-players whose versatility amounts to adding extra players to the 25-man roster-there are very few of them in MLB these days. Even Micah Owings, the rare pitcher who can hit, causes his team to question how to best use him.

While most players in MLB came from two positions as amateurs-shortstop and/or center field-the early moves to other positions around the diamond are more evolution than change. “Finding where the bat plays” is more of a concern than the defensive value in most cases, but at the major league level, players changing positions involves large decisions that are fraught with the major concern of injury risk. Last week I asked Jerry Hairston Jr. why he thinks a player is more likely to be injured when he shifts positions, and he told me, “It’s hard!” As a player that is asked to play multiple positions in a single game from time to time, let alone the six that he’ll likely play this season, Hairston understands the challenges of making the switch better than most. “Mostly it’s about comfort. If you don’t have time to take reps at all of them, you don’t get comfortable, and you start thinking out on the field.” Repetition is the key for any player, especially at the key defensive positions in the infield.

A multi-position player like Hairston is rare in that he has “plus” positions, places on the diamond where he’s considered a defensive asset rather than a passable short-term solution. Using BP’s roughest measure of fielding skill, Fielding Runs Above Average (FRAA), Hairston is roughly average at almost all of the positions that he’s played over the last two seasons. However, seeing that paired with Hairston’s extensive injury history is no surprise; he’s never allowed to attain a comfort zone at any one position, and a dig through the game logs finds that his injuries often come with in-game shifts in position.

One of the most misunderstood issues with changing positions is the risk that it entails. Over the past five seasons for which I have detailed injury data, players making a move have a 30 percent increase in the risk of injury if they’ve never played the position before. This is across the board, whether moving from something simple-like switching from left field to right-or more complicated, like going from the outfield to second base, as Kelly Johnson did a couple of seasons ago (and that after coming up as a player who had been playing the left side of the infield in the minors). It may seem counterintuitive that no single position-especially one like second base, where the second baseman’s back is to the double play in most situations-is more dangerous to move to than any other, but it does focus on the true source of the danger: unfamiliarity.

“Millions,” was the answer that Jose Reyes gave when I asked him how many grounders he took during a season at shortstop. While that might seem like an exaggeration, it’s that type of repetition that allows a player to know every blade of grass and every contour at his park. It’s that level of repetitive knowledge that really illustrates that this is the root cause, and Jose Reyes, we hope, won’t be made an example of now that his home park has changed. It’s that recent development in baseball-all of the new stadiums-that has shown that it’s knowledge of both the park and the position that helps to prevent some injuries, or rather, that the first few weeks at a new position or location are the most dangerous. It’s easy to understand for outfielders, who have to take some time to learn the wall, the feel of the warning track and the foul ground, and more, but why is it that infielders show the same risk?

One theory that seems to hold true is that fielding is instinctive. Yogi Berra said that he couldn’t think and hit at the same time; it’s perhaps more true that a player can’t think and field simultaneously. That millisecond of hesitation might lead to a collision, or being in the wrong place, or getting off to a slow start, possibly causing a muscle strain as the player tries to physically overcompensate. Any change-new position, new park-will throw that wee bit of thinking into the mix.

How long does it take for a player to get comfortable? It’s different for everyone, but it appears to take somewhere between 30 and 60 games before the risk level returns to normal. Remember, that risk does not predict a result-just because a player is more likely to be hurt doesn’t mean that he’s guaranteed to be injured. It’s likely that the position change also has an effect on defense, and even on offense, but that is more difficult to judge because of the number of complicating variables.

While players as different as Mark Teahen, Skip Schumaker, and Emilio Bonifacio attempt to make position changes, or as someone like Brian Bogusevic tries to make a Rick Ankiel-like shift from the mound to the outfield, it’s important to remember that, while Jerry Hairston says “it’s hard,” it’s likely much more difficult than that.

A version of this story originally appeared on ESPN Insider Insider.