Mets manager Carlos Mendoza looks on before a game against the...

Mets manager Carlos Mendoza looks on before a game against the Chicago Cubs at Citi Field on Thursday. Credit: Jim McIsaac

The Mets’ marathon run, likely to nowhere, reached the halfway point Thursday night at Citi Field, still 81 games to go. They were closing out a four-game series against the Cubs after their latest cringe-worthy work the day/night before.

Their reign of error in the second game of Wednesday’s doubleheader will be hard to erase from the minds of anyone who saw it, a six-miscue nightmare on the infield.

Sean Manaea, who lasted only three innings-plus, summed it up perfectly afterward by saying, “Just right now we’re playing really, really bad baseball.”

But the Mets were clinging to the hope of a U-turn despite showing no signs of being capable of consistent play and after sending another regular to the injured list and trading one of their veteran pitchers.

“The messaging is obviously that was unacceptable,” manager Carlos Mendoza said about that six-error debacle. “ ... But one thing with these guys, they’re going to show up the next day and they’re going to continue to work. But we’ve got to do it quickly here.”

Mendoza can only work with what David Stearns gave him. The roster construction mistakes and the injuries have left him short.

Pete Crow-Armstrong delivered an RBI double off Brooks Raley in the 10th on Thursday night as the Cubs finished off the four-game sweep by beating the Mets, 4-3. That extended their skid to six straight and eight of 10.

So after 81 games, they’re a season-worst 13 games under .500 at 34-47 and 9 1⁄2 games from the last wild-card ticket.

“Everybody in here is frustrated,” said Eric Wagaman, who cracked a two-run homer as a pinch hitter. “We know what our record is. We can’t get all those wins back in one day. We just have to start stacking them and take it day by day.”

The Mets hoped Freddy Peralta, a candidate to go before the Aug. 3 trade deadline, could rebound in the series finale after posting an 8.14 ERA in his previous five starts. That included 10 runs allowed in his most recent outing against the Phillies.

He was better Thursday, throwing five shutout innings before leaving with two outs and three unearned runs having scored in the sixth.

Shortstop Ronny Mauricio made a low throw to first that couldn’t be scooped by Mark Vientos, the first of two errors in the inning. The defense was guilty again.

Wagaman hit his two-run homer in the sixth and Jared Young led off the seventh with a homer to tie the score at 3, but the Mets left 14 on base.

“Losing is no fun,” Peralta said. “At the end of the day, I know that we need to win and we have a team that’s supposed to win. What I can say is we’ve been trying and we’ve been working really hard.”

Earlier in the day, second baseman Marcus Semien, who made two errors and missed catching two shallow outfield pop-ups Wednesday night, went on the 10-day injured list with a left hip flexor strain.

“This is something that he’s been battling for weeks now,” said Mendoza, who added that Semien is week-to-week.

Rookie lefty Zach Thornton will be recalled to start against the Phillies on Friday night, and Christian Scott will come off the injured list to start Saturday.

Given the way David Peterson had been going since last August, it seemed as if the Mets were going to have trouble even getting a $100 gift card back in a trade. He was 3-6 with a 6.09 ERA in eight starts and eight relief outings this season.

But Peterson was an All-Star last July. The Cubs traded for him and sent the Mets a corner infielder in Class A, Cole Mathis.

“It got to a point where we needed the flexibility on the roster,” Mendoza said. “Talking to David [Stearns], we already have Kodai [Senga] in the pen and to continue to have starters in the bullpen, it’s just not going to be sustainable. There was an opportunity to make a deal.”

The Cubs have been dealing with rotation injuries.

“We’re hopeful that we can get a starting pitcher [in Peterson] that can kind of solidify some things at a time of need for us,” manager Craig Counsell said, “and hopefully give him a fresh start and get him back to some success that he’s had.”

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