MILWAUKEE — Neil Walker left Saturday’s 7-4 loss to the Brewers with lower-back tightness, giving the already banged-up Mets more reason to worry about losing another major bat.

Walker will not play in Sunday’s series finale. Instead, he will be seen by a doctor.

Plagued with back issues in the past, Walker downplayed the tightness, but manager Terry Collins said, “Any time you talk about the lower back, I think it’s a concern.”

Walker was in the starting lineup for the first time since Thursday, when he absorbed a 107-mph one-hopper off the center of his chest. He was limited to pinch-hitting duty Friday. Walker batted twice Saturday, flying out to center and hitting into a double play.

But at the end of the third inning, television cameras spotted a clearly frustrated Walker firing his glove into the Mets’ bench. Kelly Johnson took Walker’s spot at second base in the bottom of the fourth.

“Those guys want to play, they want to contribute,” Col lins said. “They know we need them in the lineup and they don’t like to sit on the side.”

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Walker later said he felt his back tighten on a checked swing in his first at-bat, prompting him to eventually pull himself from the game.

“I know exactly what’s going on,” Walker said. “It just tightened up. Like I said, every time I’ve had something like this, it’s two or three days and I’m ready to go. But I have to fully let it calm down.”

Walker dealt with a herniated disc that ended his season in 2012. He said this issue is muscular in nature, more akin to the back tightness that cost him a handful of games in 2014.

Walker is hitting .271 with 13 homers and 25 RBIs. With David Wright, Travis d’Arnaud and Lucas Duda on the shelf, the Mets can ill afford another absence.

“I’m not concerned,” Walker said. “Like I said, I’ve dealt with this the last four or five years, so I kind of know.”

Mets videos

D’Arnaud catches

Travis d’Arnaud (right rotator cuff strain) saw action behind the plate for the first time since beginning his rehab stint with Class A St. Lucie.

D’Arnaud allowed two stolen bases (though only one chance was contested) and was lifted after seven innings. He went 2-for-4 with a walk and two RBIs, lifting his average to .273. D’Arnaud’s rehab stint expires June 24.

Extra bases

Curtis Granderson has broken out of his slump. He is 6-for-12 in his last three games and fell a double short of the cycle Saturday. “He’s starting to get on,” Collins said. “He’s starting to work the count like we know he can.” . . . Michael Conforto is 0-for-9 since homering Wednesday. His average has slipped to .234, but the Mets have yet to seriously consider a demotion to the minors.