Mets righthander Seth Lugo is headed for the disabled list to begin the season, multiple sources told Newsday on Saturday, opening the door for Rafael Montero to claim the final spot in the bullpen.

Lugo, 27, had an MRI exam Saturday that revealed no structural damage to his right arm, sources said. But the Mets intend to place him on the 10-day DL retroactively, meaning he could be activated as soon as April 10.

Once official, the move will resolve the lone question looming over the Mets’ Opening Day roster, but it will leave the team’s starting pitching depth somewhat compromised. Lugo joins lefthander Steven Matz on the disabled list.

Lugo had been experiencing what manager Terry Collins called lingering fatigue stemming from his recent workload, which included a stint with Team Puerto Rico in the World Baseball Classic. Lugo pitched 15 innings in the WBC, leading the staff. He finished with a 4.78 ERA in 26 1⁄3 innings, including tournament and Grapefruit League action.

Lugo’s work last season vaulted him into consideration in the Mets’ pitching plans. When injuries ravaged the rotation, he went 5-2 with a 2.67 ERA down the stretch.

With one open spot left in the bullpen, the Mets brought Montero and Lugo to New York this weekend. Team officials intended to watch Lugo throw a bullpen session Sunday, but those plans were scrapped.

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“That might have been where I got my arm a little tired, was just trying to do a bit too much that last outing in spring, trying to have the same intensity I had in the World Baseball Classic,” Lugo said Friday. “I think I just didn’t know that I needed to back off a little bit, focus on getting ready for the season.”

Now the Mets will carry Montero, who had an outstanding spring training in which he got himself back in the organization’s good graces.

Montero, 26, posted a 1.77 ERA in 20 1⁄3 innings, exuding a newfound confidence on the mound. He challenged hitters in the strike zone, one of the weaknesses that had kept him from establishing himself as a major-leaguer.

“Montero has experienced a kind of renaissance and pitched extremely well here,” general manager Sandy Alderson said this past week.

Montero made the Opening Day roster in 2015, part of the Mets’ stable of promising young pitchers, but he nibbled when he faced big-league hitters. Later, he spent much of the season battling shoulder inflammation.

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The injury lingered though tests showed no underlying issues, which eventually frustrated team officials. Montero pitched 19 innings in nine appearances (three spot starts) for the Mets last season, posting an 8.05 ERA. But he spent spring training this year regaining some of his lost luster.

The loser of the competition for the last rotation spot would have been sent to Triple-A Las Vegas to serve as rotation depth in case of another injury.

Losing Lugo and Matz means the Mets are thin. The only starting pitcher on the 40-man roster with big-league experience who is not already on the big-league roster is lefthander Sean Gilmartin.

Extra bases

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Zack Wheeler threw 68 pitches (44 strikes) in a camp game Saturday in Port St. Lucie and will rejoin the Mets on Sunday . . . Once rosters are submitted, five players will go on the 10-day DL: Matz, Lugo, Brandon Nimmo (hamstring), Juan Lagares (oblique) and David Wright (shoulder) . . . Utilityman Ty Kelly will be added to the 40-man roster. He will use the spot when closer Jeurys Familia begins his 15-game suspension.