Left field is generally regarded as being the less taxing defensive position: on the defensive spectrum, the only position rated easier is first base [well, and DH - but this is the National League, so we will not speak of it again]. Right field is next to it; the main thing they have to do, which left fielders don't, is throw across the park when runners try to go from first to third on a single. You'll not often see that on a ball hit to left. Hence, a strong, accurate throwing arm is more necessary in right, and seems to be a flaw of Tomas. Chip Hale said, "His arm strength is great but his accuracy wasn’t very good last season, and I’m hoping that improves."
One surprising thing I found was, RF is no less busy. My logic was, hitters pull the ball more than they go opposite field. Most hitters are right-handed, so that should mean more balls down the line to LF. However, pull tendencies are much stronger for left-handed hitters [this is why you more often see an extreme defensive shift for them, with three fielders on the right of second]. Last year in the NL, RHB pulled 102% of the number of at-bats that went the opposite way. But left-handers pulled over twice as many (209%) - they actually ended up with more pull ABs than right-handers, keeping RF busy. As a result, per team, a right fielder handles about 11 more balls in play a year.
But how well will both David Peralta and Yasmany Tomas handle the change? For it's a question that is not limited solely to Tomas, whose defensive work in right was obviously weaker. It's also worth remembering that Peralta is not a "natural" outfielder, having initially been a pitcher. We don't have figures before 2011, when he was re-inventing himself as an outfielder. But since leaving independent ball in 2012, David has primarily played in left; the main source of right field experience has been his winter-ball stints in Venezuela, where he appeared fairly regularly for Bravos de Margarita.
In terms of major-league experience, and thus any fielding metrics, we have very little to go on - only 410 total innings, barely 45 games' worth, and far short of what you need for any degree of reliability. However, with that clear small sample size warning, the figures to date are cautiously optimistic. Peralta has a UZR/150 of +12.2 in right, compared to -5.9 in left. The other metrics tend to agree: BIS Defensive Runs Saved and Total Zone's Total Fielding Runs Above Average, both have Peralta as below-average in left, but above average in right. If we break UZR down further, it's Peralta's arm in particular that is responsible for much of the difference.
What about Tomas? Apparently, the reason the team originally put him in left-field was "because that was where he said he felt more comfortable" That is where he played with the Reno Aces, before being called up last season... for all of five games. However, not long before he was signed, Kiley McDaniel of Fangraphs wrote, "his fringy to below average arm makes him a left field fit," though also said, "Some scouts said they’ve seen Tomas’ arm be solid-average at times in the past, so there’s a chance he could work in right field as well." The latter did not appear to be the case in 2015, when (in 463.2 innings, so SSS again) he had a UZR/150 of -15.6, ranking only above Matt Kemp.
Yesterday's announcement seems to be an admission that the numbers weren't lying. UZR actually had Tomas even worse in left, but with a sample size there of only 31 innings, you can discard that. The team will at least have a full spring training to get him up to speed there, and the article hints that Tomas may already be working on this, Hale calling Tomas "a full-time outfielder now in spring training" [though how you read the manager's sentence depends on whether you put the mental comma before or after "now"!] It should help having Gold Glover A.J. Pollock next to Tomas in center, and it'll be interesting to see how far over towards left-field A.J. is shaded, so that he can use his range to help out the Cuban.
Left-field has, traditionally, been a gurgling vortex of defensive such, relatively speaking, for us - this goes back to the latter days of Luis Gonzalez's career, after he separated his throwing shoulder in a collision with Tony Womack, and virtually needed a cutoff man to reach the infield thereafter. Since then, tenants have included Eric Byrnes and his flip throws, then after a brief respite into competence with Gerardo Parra, we say the likes of Jason Kubel and Mark Trumbo as everyday occupants of the spot. Peralta last year was relatively solid, but I'm going to practice my eye-rolling during spring training - I suspect I may need it if Tomas takes over down in the left-field corner.