Heading into this season, there were no guarantees that David Peterson would throw a single pitch for the Mets. The team spent its winter investing in starting pitching depth and felt good about its rotation. Peterson, the team’s top upper-Minors pitching prospect, was there in case something went wrong.

When things did go wrong, Peterson responded. He capped his rookie season with seven strong innings on Thursday, while Robinson Chirinos hit a two-run homer to keep the Mets’ microscopic playoff hopes alive in a 3-2 win over the Nationals. To qualify for the postseason, the Mets must win all their remaining games and have multiple other teams go winless.

More pertinent to the Mets’ future is Peterson, who finished his rookie year 6-2 with a 3.44 ERA. Along the way, he gave the Mets plenty of reason for optimism.

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“David Peterson’s an excellent pitcher,” closer Edwin Díaz said through an interpreter. “The stuff that he has and what he’s been able to do this year has proven he’s a big leaguer.”

The tools

Although Peterson was a first-round Draft pick in 2017, he never ranked highly on national prospect lists, in part because of his lack of velocity. The left-hander generally topped out in the low 90s in the Minors before raising eyebrows with heaters that regularly hit 95 mph this spring. It was enough for the Mets to turn to him when Marcus Stroman’s injury created a hole in the rotation, despite knowing they could retain an extra year of team control over Peterson if they kept him in the Minors longer.

If there is a concerning aspect to Peterson’s game, it is his walk rate. He issued only one free pass on Thursday, but it led to a run, as Josh Harrison hit an RBI single three batters later.

Overall as a rookie, Peterson averaged 4.35 walks per nine innings, partially by design. Part of his game is built around tempting hitters to chase; he has thrown more than 56 percent of his pitches outside the strike zone, which isn’t entirely by accident. But as he has progressed, he has grown more comfortable challenging hitters with his fastball, making his slider more potent because he’s ahead in the count more often.

“He’s getting better and better,” Chirinos said. “The first time I caught him, his stuff was good, but it was not consistent. … After that, he started getting more [over] the plate with his stuff. And his stuff is really good.”

The opportunity

Outside of Jacob deGrom, who is under contract on a long-term megadeal, the Mets have few pitching certainties heading into the winter. Stroman, Rick Porcello and Michael Wacha are all about to become free agents. Noah Syndergaard is rehabbing from Tommy John surgery, with no guarantee that he will be ready by Opening Day. Steven Matz has done little to justify a guaranteed rotation spot going forward, and the Mets have dealt away nearly all of their upper-level Minor League depth.

It is too early to know how prospective new owner Steve Cohen might operate in free agency, but even if he acquires two surefire starters, the Mets would still have rotation spots for both Seth Lugo and Peterson.

“I love to compete,” Peterson said when asked about that prospect. “I think that’s one of the biggest things that I wanted to prove to myself and to everyone else, that I belong up here.”

The hardware?

Given what Peterson has achieved as a rookie, it’s worth asking if he’s a legitimate candidate to become the Mets’ second straight National League Rookie of the Year Award winner.

Consider it unlikely, though that shouldn’t diminish what Peterson has accomplished. San Diego’s Jake Cronenworth is probably the Rookie of the Year favorite given his workload -- he’s played in 50 games -- and production on both sides of the ball. Philadelphia’s Alex Bohm has a case, as do Dodgers pitchers Tony Gonsolin and Dustin May, Marlins phenom Sixto Sánchez, Brewers reliever Devin Williams and several others.

Because the short season hasn’t given players time to separate themselves statistically as much as usual, it’s possible many rookies will receive votes, including potentially Peterson and Andrés Giménez. But the Mets won’t be evaluating their young left-hander on that. (Although Peterson finished the year with 49 2/3 innings, which normally would keep his rookie status intact for next season, a source said Major League Baseball is formulating new rules to account for the shortened season. Peterson is unlikely to remain a rookie in 2021.)

In the big leagues, Peterson hasn’t quite shown the same four-seam velocity, instead using his slider as a devilish out pitch. He has generated 18 swings and misses on his slider over his last two starts, including three of his four strikeouts on Thursday.

Regardless of how Peterson is viewed nationally, the Mets see him as a critical piece of their future.

“Peterson’s been a solid starter for us,” manager Luis Rojas said. “He did everything we asked. … It shows who this kid can be for us.”