David Peterson of the Mets pitches against the Nationals in...

David Peterson of the Mets pitches against the Nationals in the first inning during their game at Citi Field on Thursday. Credit: Getty Images/Al Bello

David Peterson penned another chapter Thursday in what’s becoming the great story of his rookie season. Except this chapter came with an ominous ending.

Peterson pitched five innings in the 8-2 win over the Nationals at Citi Field. The lefthander allowed only one hit, a Yan Gomes single in the fifth, and walked two while striking out three. Washington scored an unearned run off him in the first inning because of a Tomas Nido throwing error.

But Peterson came out of the game after throwing only 74 pitches, the fewest in his four very good starts since taking over a rotation slot for injured Marcus Stroman. It’s the fewest pitches he’s thrown and afterward he admitted to left shoulder soreness.

Asked if he was concerned, Peterson replied “not at all.”

“It’s just a little soreness. Pitchers get sore after they throw,” Peterson said. “We're managing the workload and taking care of the soreness, but it’s nothing. Nothing to worry about.”

The way things have gone for the Mets rotation this season, that’s a big ask. Noah Syndergaard was lost to Tommy John surgery. Stroman missed his first three turns in the rotation with a calf injury and then opted out of the season. Steven Matz and Rick Porcello are a combined 1-5 with a 6.92 ERA. And Michael Wacha this week went on the IL with shoulder inflammation.

Given that backdrop, Peterson has been a godsend. After Thursday’s performance, he is 3-1 with a 2.91 ERA.

“There’s a little bit of shoulder fatigue there – a little bit of that common soreness you get when you’re having (stressful) innings out there – and we played it safe,” manager Luis Rojas said. “We still have a lot of games to go. This kid is throwing the ball well. So nothing serious.”

In the second, third and fourth innings, he retired the Nationals in order. After the Gomes single, he needed to face just three more batters to finish the fifth.

“I did not want to come out of the game,” Peterson said. “Obviously, I felt good against that lineup. But we also have another month and a half of the season, so we’ve got to be careful.”