Mets starting pitcher Steven Matz leaves the field at the...

Mets starting pitcher Steven Matz leaves the field at the end of the third inning on Sunday, April 1, 2018 at Citi Field. Credit: Joseph D. Sullivan

Steven Matz wasn’t good in his first start of the season on Sunday and the Mets suffered their first defeat of the Mickey Callaway era in a 5-1 loss to the Cardinals at Citi Field.

It was a bit of a throwaway game for the team, which won two of three in its season-opening series. As Callaway said, “If we win every series, we’re going to be in a good spot.”

The positives for Matz, though, were harder to find than the Easter eggs hidden in Grandma’s backyard.

The Long Island lefty followed up a dominant end to spring training by allowing three runs in four innings and needing 89 pitches to do it. Matz walked four, including two of his first three batters, and allowed solo home runs by Paul DeJong and Yadier Molina.

“He just battled himself all [day],’’ Callaway said. “He wasn’t finishing at times and just didn’t look like he had the confidence to throw it over.”

Questioning his pitcher’s confidence reminds one of Aaron Boone saying he thought Sonny Gray was “nibbling” during a spring training outing. It seems as if our town’s new managers aren’t afraid to be honest when they see something they don’t like from one of their players.

With the Mets, accountability is in, Callaway has stressed since the day he got hired. Don’t believe it? Just ask Zack Wheeler, who fancies himself an ace-type pitcher. After a poor spring training, Wheeler indeed will be a No. 1 this season — for the Triple-A Las Vegas 51s, who open their season on Thursday against the El Paso Chihuahuas.

Wheeler thought his past promise would earn him a rotation spot as long as he proved he was healthy. Callaway and new pitching coach Dave Eiland nixed that idea, introducing the notion that Wheeler actually had to get people out to earn a trip to Flushing.

Instead, Callaway went with curveball artist Seth Lugo for the starting spot — a 34th-round pick from the 2011 draft over the sixth overall pick from 2009.

The Mets took Matz with a second-round pick in 2009. He had everything you want in a starting pitcher, with stuff that made scouts drool, and the only knock was that he couldn’t stay healthy.

Fast-forward to the present day. Matz turns 27 in May and is coming off an injury-plagued 2017 during which he went 2-7 with a 6.08 ERA in 13 starts.

Perhaps most important, the new dugout regime of Callaway and Eiland have nothing invested in the pasts of Matz, Wheeler, Matt Harvey and the rest of the Mets’ big young arms. It’s all about now.

Harvey will make his season debut on Monday night against the Phillies. All you need to know about the humbled Harvey’s status is that he is starting the fourth game of the season.

“His confidence is really high right now,” Callaway said. “He looked great in spring training and he has good stuff.”

Of his rotation as a whole, Callaway said: “I’m really excited. I think we have obviously a really good rotation to send out there and we have more than is just in our rotation right now waiting in the back. We have plenty of depth to have a great season. We have plenty of arms to get the job done.”

Plenty of arms.

Remember, someone has to go when Jason Vargas is ready, and the veteran lefty is slated to throw a simulated game to Michael Conforto on Monday afternoon at Citi Field. Once Vargas’ surgically repaired non-pitching hand is healed, he will join the rotation.

What if Lugo outpitches Matz, who still has minor- league options? A few more outings like Sunday’s and Matz could end up catching the red-eye to Vegas, and it won’t be a pleasure trip.

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