PHOENIX -- The Mets demoted Kirk Nieuwenhuis to Triple-A Buffalo after Sunday's 5-1 win against the Diamondbacks to clear a roster spot for the return of Mike Baxter from the disabled list for the series opener in San Francisco. Baxter had been sidelined since suffering a separated shoulder on a June 1 catch that preserved Johan Santana's no-hit bid.

Nieuwenhuis, who had joined the Mets right after Andres Torres suffered a calf strain on Opening Day, was hitting .252 with seven homers and 28 RBIs in 282 at-bats. He had a team-high 98 strikeouts. Nieuwenhuis was only 9-for-his-last-73.

His demotion comes five days after Lucas Duda also was dispatched to the Bisons.

Hitting coach Dave Hudgens noted teams have exploited Nieuwenhuis by putting breaking balls in the dirt, then throwing high fastballs above the strike zone that Nieuwenhuis was prone to chase.

"He chased some a little bit. He's got to learn to lay off that fastball up," Hudgens said. "It's a ball for the most part. They'll try some soft stuff down, and then they try to elevate the fastball. It's just an adjustment he's going to have to make. They try that on a lot of guys, and you've got to be disciplined. It's tough, because you see that ball so good. It's close to your eyes and you see it good, so you react. You just have to learn to lay off it, because you can't catch up to it."

Said Nieuwenhuis: "Right now I'm not contributing. It's something you want to do. It's something that you need to do. Hopefully I'll get it going down in Buffalo, and hopefully be back soon."

Nieuwenhuis' highlights included an April 26 walk-off RBI single with two outs in the ninth against Heath Bell as the Mets completed a sweep of the Marlins.

One positive byproduct of the demotion from an organization perspective: If Nieuwenhuis spends 20 consecutive days in the minors, he will not be eligible for free agency until at least after the 2018 season, rather than after the 2017 season. That's because he would not be credited with a full year of major league service time for this season.

"There's something to be said about contributing to the team," Nieuwenhuis said. "And that's really, ultimately what it's all about. And putting W's up on the board. Right now I'm not helping."