"He's playing left field, which is a difficult position to play. At some point, we may have to see if he is a better right fielder. If he is a better right fielder, that may give him peace of mind and comfort, which will make him a better hitter. We have to look at all possibilities to put our players in the best position we can to get the best we can out of them."
-- Dave Stewart
General baseball wisdom would disagree with Stewart's first sentence in particular. Bill James came up with the idea of the "defensive spectrum", which arranges the positions like colors of the rainbow, from easiest defensive position to hardest. Players can generally move to the left - to easier positions - without too much difficulty, but it's rarer to see a player go the other direction. The generally accepted spectrum is:
DH < 1B < LF < RF < 3B < CF < 2B < SS < C
According to this, left-field is "a difficult position to play" only in comparison to first-base. This also suggests that an equal alternative to moving Trumbo to right, might be moving Paul Goldschmidt to left, and letting Trumbo stay at first. I think Goldie has the athleticism, and certainly the work ethic, to make a decent stab at it, and I'm sure he would do so without a whisper of complaint. However, I doubt the team would go there. On the other hand, while uphill, right field isn't that far from left on the spectrum - it's not as if the team is contemplating having Mark turn double-plays up the middle with Didi.
How much of a difference is there between playing left and right? The obvious difference is that the latter needs a better arm, to make the longer throws to third-base; left-fielders hardly ever have to throw to first. That may play to Trumbo's strength, since over his career, his arm has come in slightly above average (+2.7 runs). But there is more to it: over on Beyond the Box Score, Ryan Morrison looked at the differences between left and right, and found the latter also get more chances, by about 10% [though both trail well behind CF]. That would hurt Trumbo, since his main defensive issue is range [-14.0 runs]
When players move from LF to RF, how does their defensive prowess stack up? To try and answer the question, I looked at those who played both positions in the same season over the past five years. I set the minimum at each spot to 40 games, about one-quarter of the season, and that game me a pool of 24 players to work from. Now, obviously, defensive metrics are highly volatile, and individual stats are likely to vary, just on chance. But by grouping them together, we get a pool of just over a thousand games worth in left and about 1,130 in right, so that should help smooth out those random fluctuation.
What do we see? Not a great deal of difference overall: these players had a UZR per 150 games of -2.6 in left, and -2.9 in right. However, it's worth remembering that these figures are compared to the average at that position. Right fielders are generally better than left fielders at converting balls into outs (RZR, as it's known). Last season, LF converted 88.4% of balls hit into their zone to outs; RF converted 90.2%. Given this likely higher baseline, it certainly doesn't seem there is much, if any, negative impact on a player's defensive prowess, if he moves from left field to right during a season.
However, nor is it likely to turn Trumbo into a Gold Glover. The first column above is total dWAR [which also includes value at any other positions] and the table is in descending order of that. What we also see is that good defenders overall, are likely to be good defenders at both right and left. There were only four players with a positive dWAR, but they were also in positive territory for UZR at both corner outfield positions. Conversely, the last six names at the bottom of the chart, are all below average in both spots. While the middle has some wild differences - Gabe Gross in 2010 was a traffic bollard in left, but Gerardo Parra in right - those may well be the result of small sample size.
Checking the previous studies, the above is in line with what others seem to have found. Jeff Sackmann at The Hardball Times concluded, "there’s no discernible difference in quality between average left field and average right field performance." Mitchel Lichtman, the inventor of UZR, concurred: "It probably makes almost no difference, range-wise, where you play your corner outfielders. You simply choose your two worst OF’ers, and put the one with the strongest arm in RF." So in all probability, the move won't end Operation Trumbo Drop - just relocate it for the enjoyment of a different section at Chase Field.
But here's one final idea, first floated by Eno Sarris: why not flip LF and RF depending on the hitter? Right-handed hitter with a heavy pull tendency in the air? Have your better corner outfielder in left, because that's where the ball is more likely to be hit. As infield defensive shifts become increasingly prevalent, some enterprising team may adopt this tactic for the more extreme batters. Or remember that game earlier this year against Houston where we saw Tony Sipp playing right-field, so he could come back to the mound and face another left-hander? I like that kind of thinking outside the box. And if Trumbo feels more comfortable in right than left, it's okay by me.