SAN FRANCISCO -- When David Peterson left a slider hanging belt-high in the middle third of the strike zone Saturday afternoon at Oracle Park, he succumbed in a way that Joey Lucchesi had not over seven innings the previous night. The juxtaposition was stark, and it could have ramifications on the future of this Mets rotation.

Brandon Crawford hit Peterson’s pitch for a three-run homer that nearly reached McCovey Cove, sending the Mets to a 7-4 loss -- just their second defeat over nine games in California so far. Over five innings, Peterson allowed seven earned runs, bloating his ERA to 7.36. The Mets’ most trusted depth starter at the outset of Spring Training, Peterson is now in danger, over the coming weeks, of being the first healthy pitcher to lose his grip on a rotation spot.

“He’s a huge competitor. He wants to win every single time he goes out. He’s a bulldog on the mound,” said outfielder Brandon Nimmo, whose homer was one of the lone bright spots for the Mets. “I know that he feels terrible about it, and I don’t ever question his want and his will to win. That just happens sometimes. It’s part of playing the game at this level.”

From the outset on Saturday, nothing worked for Peterson, who allowed two singles and a walk to load the bases with one out. David Villar followed with a sacrifice fly before Crawford bashed his homer, ensuring this would not be Peterson’s first quality start of the season. The left-hander allowed two more runs in the second inning and a Wilmer Flores solo homer in the fifth, which rendered the Mets’ late rallies mostly futile.

In particular, Peterson said, he has been struggling with his slider, which is usually his best pitch -- one he used to great effect in producing a 3.83 ERA over 105 2/3 innings last season.

“He’s had some good moments,” manager Buck Showalter said. “He’s had some good sequences and good innings. He hasn’t been quite as consistent yet, but that tells me there’s better days ahead for him, because he’s shown us what he’s capable of doing.”

In isolation, Peterson’s current struggles would be cause for concern. When considered amidst the greater landscape of his team’s rotation issues, however, they’re a legitimate problem.

Until Max Scherzer returns from his sticky substance suspension on May 1, the Mets must roll with their current rotation of Kodai Senga, Peterson, Tylor Megill, Lucchesi and a mystery starter. (José Butto is not eligible to return from the Minors until April 28, but the Mets could call him back up if they place another player on the injured list between now and then.) That means Peterson will receive at least one more start on Friday in a key NL East showdown against the Braves.

Beyond that, nothing is guaranteed. Three days after Scherzer’s reinstatement, Justin Verlander could return from his spell on the IL, requiring the Mets to remove multiple pitchers from their rotation. One subtraction is obvious: whomever the team recalls to start on Tuesday. The other probably would have been Lucchesi, had everyone been pitching to the backs of their baseball cards.

But Lucchesi’s dynamic performance on Friday night -- seven scoreless innings, nine strikeouts, the finest start of his career -- began to tip the equation in an unexpected way. Peterson’s continued struggles have tipped it further. Much can change, of course, over the next rotation turn. Perhaps Peterson rights himself. Perhaps Lucchesi falls back to earth. Perhaps another injury or issue surfaces, creating spots for everyone.

All Peterson can do is take the mound again and hope to correct his course.
 
“We’ve got some guys down,” Peterson said. “And so as a part of this rotation, I need to be better and hold my end of the bargain.”