Lucas Duda’s name was in the lineup for the Mets’ game against the Brewers on Saturday at Citi Field. And then it wasn’t.

Terry Collins had him playing first and batting sixth, but the manager had to scratch Duda because of a continuing back problem.

Stiffness had developed during the finale of the Mets’ road trip in Colorado last Sunday, so Duda received a shot Monday, then sat Tuesday and Wednesday. He returned Thursday for the Washington series finale and played against Milwaukee in the series opener Friday night.

“Well, it’s pretty concerning,” Collins said before Saturday’s 5-4 win. “He’s tried to go out the last couple of games and play, and it’s still bothering him. We had him examined by a doctor today. It was recommended he take a couple of days.”

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Asked if it’s a disc issue, Collins said, “Could be.” Duda was sent for an MRI. Collins said after the game that there was “some concern there” and that Duda will need time. He was scheduled to undergo other tests and the team expects to know more Sunday.

Duda experienced lower- back stiffness last season and was on the disabled list from Aug. 22 through Sept. 6.

“Having him out of the lineup is a huge loss for us, especially against righthanded pitching,” Collins said. “ . . . But we’ve got to look at the big picture, and that is we can’t aggravate this thing to where it may come up to be a major issue. Not that it won’t. But we just hope that it won’t.”

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Duda didn’t come out to talk about the situation after the game. The streaky lefthanded hitter is 7-for-41 in his last 13 games and is hitting .231. He’s off to a dreadfully slow start against lefties, going 1-for-25 against them for a .040 average. But his home run total across the previous two seasons is 57 and his RBI total is 165.

“Right now, seven homers and [19] RBIs are out of the middle of your lineup,” Collins said. “But I’ve always thought these are opportunities. Somebody’s got to step up and do the job.”

Wilmer Flores has made two starts at first base, but he’s on the DL and isn’t eligible to return until Friday. Alejandro De Aza also has worked out at first recently.

Eric Campbell stepped into the role of Duda on Saturday, making his sixth start at first, and had a productive day with two singles, including a leadoff hit in the ninth that triggered the game-winning rally.

“My only goal when I get a chance to play is to do something to help the team win,” Campbell said. “To get on there in the bottom of the ninth is what we needed . . . Hits bring confidence.”

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The 29-year-old infielder/ outfielder is up to .231 after 39 at-bats. He appeared in 156 games in the previous two seasons with the Mets, batting .263 and .197 in limited action.

Collins said Campbell has had to adjust to being a backup. “When Eric plays a little bit more, he shows you what he can do,” Collins said. “He’s a nice extra player on your team because he can play around, plays a lot of positions . . . He’s a solid player.”

If Campbell has to replace Duda for a bit, he will embrace it. “It’s a good opportunity,’’ he said. “I’ll be ready when my name is called on.”