Duda was hitting a disappointing .241/.335/.391 in 355 plate appearances over 89 games this year. That's basically about league average production: by OPS+ he was at 101, while by wRC he was at 103, where 100 is league average in both cases. A year ago, he hit .292/.370/.482. This year however, is isolated slugging dropped from .190 to .150, his BABIP slipped from .326 to .304 and most damning, his strikeout rate jumped from 16% to 26.5%. So, he hit for less power, and made less contact.
The problem is that he was giving away runs defensively. Lots of them. By UZR he was -17.6 runs below average, by Defensive runs saved he was -16 and by bRef Fielding Runs he was -16. Using partial-season fielding stats is usually an awful idea because the samples are so small. Three-season averages are far preferable to one-year rates. However in this case, there is a remarkable degree of agreement: Duda was a terrible rightfielder.
At 26, Duda should be peaking as a baseball player. Instead, he's moving in the wrong direction.
In related news, earlier his summer, former Mets VP of Player Development Tony Bernazard singled Duda out as a player whose progress made him feel good. From David Lennon in Newsday, Bernazard, "is especially impressed by Lucas Duda, a player he said was moved from first base to the outfield at his urging."