Lucas Duda, the Mets’ slugging first baseman, could be out for the rest of the season with lingering issues from a stress fracture in his back.

“It doesn’t look good,” general manager Sandy Alderson said on Wednesday, before a Subway Series game against the Yankees, when asked to handicap the first baseman’s chances of a return.

Duda, who has been out since May 23, had been doing baseball activities in preparation for a minor league rehab assignment. But late last week, Duda’s back flared up. Dr. Robert Watkins, the same back specialist who has worked with David Wright, recommended that Duda be shut down for a month.

That leaves only a narrow window for Duda to rejoin the Mets, who have concern that the injury lingers into next season. “It’s always a concern when a player is not able to come back late in the season and demonstrate he’s heathy going into the following season,” Alderson said. “But I don’t concern myself with that now. We’ve got other things to deal with. We’ll know a little more in the next 30 to 45 days.”

Duda 30, was hitting just .231 with seven homers and 19 RBIs when he went down.

Veteran James Loney has given the Mets a capable fill-in. More a contact hitter at the expense of power, Loney is batting .289 with six homers and 23 RBIs.

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Nevertheless, Duda’s presence has been missed in a Mets lineup that began the day 13th in the NL at 3.71 runs per game. From 2014 to 2015, Duda hit 57 homers with an .834 OPS, despite his streakiness at the plate.

Extra bases

Jose Reyes is expected to come off the disabled list when he’s eligible on Aug. 11, Alderson said. The infielder has been sidelined with a strained left intercostal (side) muscle . . . Zack Wheeler’s rehab assignment begins Saturday with Class-A St. Lucie . . . Alderson said the Mets will continue to look for relievers in potential waiver deals, the same method in which they wound up with Addison Reed last season.