The Mets swapped one struggling reliever for another before Tuesday’s series opener against the Padres, sending Hansel Robles to Triple-A Las Vegas and promoting lefthander Josh Smoker.

Robles had been expected to be a key cog in the bullpen. But staggering inconsistency caught up to the hard-throwing righthander, whose ERA jumped to 6.23 after allowing 12 runs in his last three appearances. That spike followed a stretch in which he tossed 14 straight scoreless innings.

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“When he was going through his good streak, he was locating everything,” manager Terry Collins said. “He was locating his slider, he was locating his fastball to both sides of the plate. Right now, he’s doing neither.”

Smoker, who allowed a solo shot on Tuesday, was sent down on May 9 after his ERA soared to 7.88. But the reliever started two games in Triple-A Las Vegas, with the Mets hoping the change in role would force him to use his slider and changeup more often.

Extra bases

Outfielder Yoenis Cespedes (hamstring) could be playing in rehab games in the next few days, marking the next hurdle he must clear before returning to the Mets. “I’d like to put a date and a timetable on it but he’ll dictate that with how he feels after he starts playing,” Collins said. “I’m hoping that certainly in the next few days he’ll be ready to come back.” . . . Rightfielder Jay Bruce left Tuesday’s game in the sixth inning with back tightness. He hopes to return to the lineup on Wednesday. “Hopefully it’s just an isolated incident,” he said . . . Robert Gsellman returns to the rotation on Wednesday after making two appearances out of the bullpen . . . Seth Lugo allowed seven runs in three innings in a rehab outing for Class-A Port St. Lucie . . . Steven Matz gave up five runs and five hits in four innings for Triple-A Las Vegas. Travis d’Arnaud went 0-for-4 and caught nine innings in the same rehab game.