PORT ST. LUCIE, Fla. — Tim Tebow made his first rookie mistake even before stepping into the batter’s box.

The New York Mets newcomer walked behind home plate and took his practice swings near Boston’s on-deck circle.

“I didn’t know who that was back there. I thought it was the ball boy,” AL Cy Young Award winner Rick Porcello said.

Tebow’s debut as a big leaguer didn’t go much better Wednesday.

The former NFL quarterback went 0 for 3, twice looking at strike three and also grounding into a double play with the bases loaded in a spring training exhibition against the Red Sox.

Tebow did safely reach when he was hit by a pitch in the right shoulder. But his stay on base was brief – he got doubled off first on a line drive.

“It was a first day for me getting to compete. I’ll learn a lot from it. It’s kind of what I expected from a competition level,” he said.

The 29-year-old Tebow batted eighth as the designated hitter. Signed last fall, he’s in camp on a minor league contract, hoping to make it as an outfielder. He’s next scheduled to play for the Mets in a split-squad game Friday against Houston, and Manager Terry Collins said Tebow would be in the field.

To say Tebow’s first game was a success would be a stretch. He did, at least, get high-fives on the field after an 8-7 win in front of 6,538 fans.

“With almost anything I do, I get a little nervous because I care about it, the outcome and my teammates,” Tebow said. “But I’d also get nervous if I was going to talk to a high school football team before a game.”

Mets starter Noah Syndergaard threw 47 pitches and lasted 21/3 scoreless innings.

Porcello, the Red Sox starter, was struck on the pitching hand by Yoenis Cespedes’ fourth-inning grounder.

Porcello threw a couple of pitches, and the 28-year-old right-hander tried to finish the inning.

“It’s fine right now,” Porcello said. “It happens quickly, so you’re just trying to get the sting out of it. I was able to throw a couple of pitches, and the second warmup pitch I was able to get down in the zone, and didn’t think it would affect me making pitches.”

Porcello gave up a homer to Jay Bruce and a double to Lucas Duda, then left the game.

“I felt really good,” Porcello said. “The ball was coming out of my hand a lot better. I’ve got a couple of mechanical things I’m still battling.”