It wasn’t all that long ago that David Peterson looked like an ace. The pitching staff’s workhorse, Peterson could be counted on every 5-6 days when the rest of the rotation crumbled around him this season.

Now, he’s hit a slump of his own, and it couldn’t come at a worse time.

The left-hander gave up a grand slam to Manny Machado in a 7-4 loss to the San Diego Padres on Wednesday night at Citi Field.

The series is even at 1-1, with right-hander Jonah Tong looking to secure the series win for the Mets (78-74) on Thursday afternoon.

Tied at 2-2 in the fifth, Peterson’s command wavered. He hit a batter, walked a batter and gave up a bunt single with one out. With the count full to Machado, the third baseman swatted a full-count curveball, nearly driving it to the second deck in left-

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