The Mets will have the following outfielders on the roster: righthanded-hitting Jason Bay and Scott Hairston, switch-hitter Andres Torres and lefties Jordany Valdespin, and Mike Baxter.
First, examine the overall shape of the Mets' outfield production this year.
Since April 27, in his last 73 games, including 54 starts, Nieuwenhuis has struggled in a big way. He's hit .230/.291/.338 with 79 (!) strikeouts in 247 plate appearances for a 32% strikeout rate. He has not hit lefthanders at all, going .180/.286/.230 in 73 PA against southpaws. That's the bad news. The good news is that he's still 24 years old - he won't turn 25 until August - and he's played solid if unspectacular defense.
Nieuwenhuis' immediate replacement is Whitestone's own, 27-year old Mike Baxter, who made the no-hitter saving catch for Johan Santana. Examine Baxter's production above carefully. He owns a wildly unsustainable .447 batting average on balls in play. Subtract 100 points off that to a reasonable, but still high level, and his overall line would crater. In 123 career Major League plate appearances, Baxter has hit .280/.358/.467 with a .372 BABIP. Baxter has exactly ZERO major league hits off a left-hander in 13 plate appearances, but has drawn two walks. On the plus side, he's hit .313/.382/.521 in his other 110 PA versus righties. He is not as good a defensive outfielder Nieuwenhuis.
You know what else is unsustainable on that chart above? All of Jordany Valdespin's big league production. The 24-year old has a K/BB rate of 10 (20 strikeouts/2 BB) in 111 PA. And that's the bigger problem, he's drawn TWO walks for a 1.8% walk rate. Valdespin has certainly done damage against right-handed pitching with 11 extra-base hits in 89 PA against them, he has a .581 slugging percentage. He has just two extra-base hits, both doubles against lefties in 21 PA for a .350 slugging percentage.
Scott Hairston has the reverse problem. He list lefties (.315/.346/.621 in 130 PA), but not righties (.196/.248/.382 in 109 PA) in 2012.
Lucas Duda is already back in the minors.
Jason Bay has been one of the worst position players in baseball. By weighted runs created plus (wRC+), where 100 is league average, among all outfielders with more than 100 plate appearances, Bay is the seventh-worst. He's 0-for his last 22. He has no defensive value. Bay is owed $16 in 2013, the pro-rated portion of $16 million this year with a $3 million buyout due to avoid his $17 million team option in 2014. Bay's contract, and the fact that he was a valuable big league player from 2005-2009, is keeping him on the roster at least for another week or so.
Adres Torres is hitting .235 and slugging .310. In his favor, he has a .341 on-base percentage, and plays the best centerfield of the group. He's making $2.7 million and is arbitration eligible for the third time after this season. Torres' ability to play centerfield is keeping on the roster.
Nieuwenhuis, strikeouts and woes against lefties and all, is a better option in the big leagues in 2012 than Baxter or Valdespin. The only viable explanation for sending him to AAA now is developmental. Expecting production on both sides of the ball, from Valdespin and Baxter, that exceeds what Nieuwenhuis can provide is simply not reasonable. Moreover, if this is about building past 2012, Nieuwenhuis has a far better chance to contribute to the next Mets playoff team than Baxter, Torres and Bay. The last two months obscure this fact, but Nieuwenhuis has a chance to be part of the future.
Nieuwenhuis simply needs the exposure to big league pitching. In his last tour of duty in Buffalo, he beat up International League pitching at a .298/.403/.505 rate in 2011.
While the bullpen draws much of fan ire around the 2012 Mets, the outfield's woes are nearly as severe. As a unit, the team's outfield is 22nd overall in wRC+ and 29th in UZR. Put simply, it's a bad offensive unit, and one of the worst defensive groupings. Removing Duda will improve the outfield defense moving forward. Removing Nieuwenhuis will not solve any problems and does not move the Mets appreciably forward to their next playoff team.