PORT ST. LUCIE, Fla. -- During the earliest games of spring training, as players work through the rust of the offseason, teams often hold back on making evaluations. But with so many questions to answer before Opening Day, the Mets might not have the luxury of easing into some critical decisions.

"Any time you're on the field, you're being evaluated," manager Terry Collins said Saturday, when the Mets opened Grapefruit League play with a 5-3 win over the Nationals. "I don't care what the situation is. So they have to take that into consideration."

In his first appearance with the Mets, Zack Wheeler tossed two shutout innings while upstaging Nationals pitching phenom Stephen Strasburg, whom the Mets tagged for two runs. "He's got some good stuff, definitely," Nationals slugger Bryce Harper told reporters shortly after grounding out in his only at-bat against Wheeler.

Indeed, Wheeler drew plenty of attention. The Mets also received encouraging signs in other critical areas.

Kirk Nieuwenhuis and Ruben Tejada remain in the mix to lead off for the Mets. Both made early statements, getting deep into counts against Strasburg. Nieuwenhuis led off the first inning with a single and scored on Tejada's two-run blast.

The homer was particularly rewarding for Tejada, whose emphasis throughout spring training has been improving his plate discipline.

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"Pretty good at-bat," said Tejada, whose homer came after he worked the count full. "We're looking for that. Take a lot of pitches in every at-bat and try to make good contact."

In his first action with the Mets, veteran Marlon Byrd went 2-for-2, making a strong first impression in his effort to win a spot in rightfield. Centerfielder Collin Cowgill followed suit; he was 1-for-1 and scored on a smart baserunning play.

New closer Bobby Parnell worked through a jam in the ninth. Veteran closers often pitch in the early innings in spring training, but Collins wanted to replicate the regular season. That's why he took the unusual step of saving Parnell until the end. "I just think he needs to get used to it," Collins said after Parnell earned the save.

But not all signs were encouraging. Leftfielder Lucas Duda, who has spent much of camp working on changes to his swing, was 0-for-3 with a pair of strikeouts.

Most hitters ease their way into game action, but Collins said Duda will be given extra plate appearances to work on his swing. Duda will play in Sunday's split-squad game against the Astros -- another unusual step for this early in the schedule.

"He looked a little uncomfortable today with it," Collins said. "And I want him to get more comfortable in the batter's box, so I'm going to try to get him a lot of at-bats early."

Veteran starter Shaun Marcum allowed three runs and four hits in two innings, including a two-run homer by Chris Snyder. Marcum tossed a scoreless first before allowing three runs in the second, when he lost the feel for the changeup, his most important pitch.

Said Marcum: "It's a good thing it's spring training."