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In a stretch of three straight series against difficult lineups, the Mets valued a second left-hander in the bullpen over keeping Dedniel Núñez in the major leagues.

José Castillo was added to the major league roster Saturday, with Núñez optioned to Triple-A Syracuse.

The lefty Castillo was acquired in a trade with Arizona two days earlier for cash considerations.

“It definitely helps, especially when you are going against a lineup like this or Boston or the Dodgers coming up,” manager Carlos Mendoza said before the Mets beat the Yankees 3-2. “This is a guy that when healthy, he’s got the talent. … We feel there’s more in the tank there.”

The Mets have been in search of lefty relief options since losing A.J. Minter and Danny Young to season-ending injuries.

Génesis Cabrera has provided a boost from the left side in his four appearances since arriving from Syracuse.

Last season, Núñez emerged as a key midseason piece of the bullpen, but he pitched only once over the final two months because of a forearm strain.

Núñez was late in building up in spring training, prompting the Mets to place him with Syracuse to begin the season.

Núñez pitched to a 7.36 ERA in his five appearances for the Mets after he was recalled from Syracuse on May 5.

The right-hander struggled with walks in his first two appearances, but over his final three issued only one walk.

He pitched a scoreless eighth inning on Friday.

“Even though the past [three] outings he’s been better, we just need to see consistency with strikes,” Mendoza said. “That’s what made this guy elite last year and he became who he was last year for us.”

Ronny Mauricio was reinstated from the injured list and optioned to Triple-A Syracuse.

The infielder previously was on a minor league rehab assignment in his return from surgery to repair a torn anterior cruciate ligament in his right knee.

Mendoza said the organization needs to see Mauricio built up to the point he’s playing multiple straight days and full games.

“He’s not there yet, so we have got to get him there,” Mendoza said. “It’s more about getting everyday reps, everyday at-bats, and we’ll go from there.”

Paul Blackburn extended to 73 pitches in his rehab appearance for Double-A Binghamton and will receive one additional start in the minors before he’s considered for return from the IL, according to Mendoza.