ATLANTA - Although the Mets see plenty of promise in righthander Rafael Montero, they know certain lessons must be learned on the job.

That's why they spent much of Saturday trying to understand what possessed Montero to throw nothing but fastballs Friday night.

"Hopefully, it doesn't happen again," manager Terry Collins said Saturday night, one day after Montero's bizarre outing.

In Friday night's 5-3 loss, the 24-year-old Montero was entrusted to hold the line with the score tied 3-3, but the Braves tagged him for two runs in the eighth. As the pressure mounted, Montero stuck with his fastball, even as the Braves made solid contact.

Montero threw 35 pitches -- all of them fastballs.

"You've got to show them something else," Collins said. "These are big-league hitters."

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Collins and pitching coach Dan Warthen assigned the blame to not only Montero but to catcher Travis d'Arnaud, who called for the fastballs. But Warthen said Montero could have been more assertive by shaking off d'Arnaud.

"He has to learn to shake off and call his own pitches," Warthen said. "A lot of times, that has to do with youth."

Swinging at strikes

Wilmer Flores' defense at shortstop already has drawn plenty of scrutiny, but his offense also has been a concern. After going 0-for-2 Saturday night and hitting into a double play with the bases loaded and none out, he is hitting .125, with more errors (three) than hits (two).

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Hitting coach Kevin Long tracks swings on pitches out of the strike zone. By his count, Flores has chased more than anybody else on the team.

"When you do that, that's the thing I've got to make sure of, that he continues to swing at strikes," Long said. "You're not going to hit if you expand and swing at balls."

Long spent part of Saturday encouraging Flores to adopt a straighter posture in his stance. The change is designed to discourage Flores from leaning over the plate; when he does that, it makes him more susceptible to chasing pitches.

"If he stays in the strike zone, he'll be fine," Long said. "So you've really got to make sure he stays in the strike zone."

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Extra bases

Ruben Tejada will start at shortstop Sunday against Braves lefty Alex Wood. Flores will get a scheduled day off. Tejada grounded out Saturday night in his first 2015 plate appearance.