WASHINGTON — “I knew there was nothing happening in there.”

That was Noah Syndergaard’s explanation late Friday night of why he “refused” to have an MRI a day earlier after he was scratched from a start because of right biceps soreness.

On Sunday, Syndergaard’s decision may have proved to be detrimental to himself and his team. And as the day ended, Syndergaard was headed back in New York to have an MRI Monday morning — because something was happening somewhere — and third-string catcher Kevin Plawecki was pitching the final two innings of the Mets’ 23-5 loss to the Nationals.

“I think he understands that there’s something going on now that he needs to have examined,” general manager Sandy Alderson said.

An ineffective Syndergaard left with one out in the second inning after throwing a 90-mph changeup to Bryce Harper and grabbing his right side in pain. Syndergaard immediately stumbled off the mound, removed his glove and flexed his right arm. His left hand rested under his right armpit as manager Terry Collins and trainer Ray Ramirez raced to the mound.

Seconds later, Syndergaard was headed to the dugout with what the Mets termed a “possible lat strain.” Syndergaard left Nationals Park during the game and was unavailable to discuss the turn of events that probably has him heading to the 10-day disabled list.

advertisement | advertise on newsday

“[The lat strain] may or may not be related to his previous complaint,” Alderson said.

Asked if Syndergaard expressed any regret at not getting an MRI earlier, Alderson said: “We didn’t get into that. I didn’t think that was necessary at that particular time.”

Alderson said Syndergaard’s successful bullpen session Friday convinced the Mets that he was healthy enough to pitch. Syndergaard was throwing hard Sunday — several 100-mph fastballs — but still allowed five runs and five hits in the first inning.

Syndergaard, who had not pitched since April 20, threw 38 pitches. Plawecki, who said he hadn’t pitched since he was 14, threw 32.

Anthony Rendon, who entered the game with zero home runs, went 6-for-6 with three home runs and a Nationals-record 10 RBIs as Washington hit seven home runs (three off Plawecki) to salvage the finale of the three-game series.

Mets videos

The Mets, who had lost six in a row before this series, were feeling pretty good about themselves after beating Max Scherzer and Stephen Strasburg in the first two games. Now they have fresh worries about their ace and a fresh round of questions about how Syndergaard and the team handled the initial injury and whether that led to Sunday’s debacle.

“We asked him how he felt and he felt fine,” Alderson said. “Said he could have pitched on turn last time out. And we took him at face value, and he also threw a [bullpen session] and felt fine. On the basis on that input as well as his own comments, he was good to go.”

And then he went. The Mets trailed 5-1 when Sean Gilmartin replaced Syndergaard, and they came close to making a game of it, pulling to within 6-5 by the fourth. Rene Rivera and Jay Bruce hit solo homers, Gilmartin had an RBI double and Michael Conforto hit an RBI single off starter Joe Ross. But Gilmartin allowed four runs in the fourth and Fernando Salas gave up three in the fifth as the Nationals took a 13-5 lead.

Rendon, who had a two-run single off Syndergaard in the first, hit a solo home run in the third, a three-run shot in the fourth, a three-run double in the fifth that was about a foot from being a grand slam and a solo homer off Plawecki in the eighth.

After Matt Wieters hit a three-run homer off Josh Smoker with none out in the seventh to make the score 19-5, Plawecki made his major-league debut as a pitcher. He retired three batters on long fly balls. In the eighth, he gave up a leadoff homer by Harper, a two-run shot by pinch hitter Adam Lind and a solo homer by Rendon.

advertisement | advertise on newsday

Plawecki had a smile in the postgame clubhouse. Collins most certainly did not.

Asked if he was as angry as he looked in the dugout, a red-faced Collins snapped: “You think? What do you think?”