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.

The former NFL quarterback’s debut as a big leaguer didn’t go much better Wednesday.

Tebow 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.

The 2007 Heisman Trophy winner did safely reach when he was hit by a pitch in the right shoulder from fellow Florida alum Brian Johnson. But Tebow’s 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,” Tebow said after an 8-7 win.

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.

“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.”

Tebow was set to lead off the third inning for his first at-bat. The lefty hitter emerged from the Mets’ third base dugout and crossed over to the Boston side, drifting toward the Red Sox on-deck circle and inching his way into Porcello’s view.

“I thought you walked around because you’re a left-hander. I found out you don’t do that,” Tebow said.

Porcello threw a fastball past Tebow, then got him to look at strike three. Tebow had a smile and friendly word for plate umpire Ryan Additon after getting called out.

Porcello was struck on the pitching hand by Yoenis Cespedes’ grounder in the fourth. The righty stayed in, but exited after Jay Bruce and Lucas Duda doubled.

“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 was 22-4 with a 3.15 ERA last season.

AROUND THE GRAPEFRUIT AND CACTUS LEAGUES

METS (ss) 8, RED SOX 7

Mets starter Noah Syndergaard threw 47 pitches and lasted 2 1/3 scoreless innings, and Bruce hit his first home run of spring training and had three RBIs.

CARDINALS 9, NATIONALS 3

Stephen Strasburg was hit hard in his second start, giving up six runs and seven hits in two innings. Jhonny Peralta hit an RBI groundout in the first, and Matt Adams, Patrick Wisdom and Harrison Bader followed with run-scoring singles. Aledmys Diaz had an RBI double in the second and scored on Adams’ sacrifice fly. The Cardinals’ Michael Wacha allowed two hits in three scoreless innings.

ASTROS 12, METS (ss) 2

Houston said 2015 AL Cy Young Award winner Dallas Keuchel pitched a simulated game Tuesday with no issues and he is to make his exhibition debut Sunday against Miami. The left-hander did not appear in a game after Aug. 27 last year because of pitching shoulder inflammation. Against New York, starter Charlie Morton struck out four in three hitless innings. Mets starter Robert Gsellman allowed two runs and five hits in five innings, and Asdrubal Cabrera homered off Tony Sipp.

YANKEES 10, CANADA 4

Former AL MVP Justin Morneau had a single and struck out twice for Canada, and Seattle’s Tyler O’Neill hit a two-run homer. Matt Holliday hit a two-run homer and Gary Sanchez had a solo drive for the Yankees.

DOMINICAN REPUBLIC 10, PIRATES 6

Baltimore All-Star Manny Machado hit a two-run homer for the Dominican Republic. Gregory Polanco, who plays for the Pirates, went 3 for 3 for the DR, and Starling Marte added two hits and two RBIs. Pirates starter Gerrit Cole allowed two hits in three shutout innings.

BRAVES 3, PHILLIES 2

Bartolo Colon allowed one run and one hit in three innings, striking out three, and Nick Markakis hit a two-run double.

Jeremy Hellickson gave up two runs and five hits in four innings, leaving him with a 5.00 ERA.

ORIOLES 6, BLUE JAYS 4

Craig Gentry, battling for a roster spot, hit a three-run homer as part of a two-hit day. Mat Latos, trying to earn a job with Toronto, allowing five runs, five hits and three walks in two innings.

A’S 7, ITALY 0

A’s starter Kendall Graveman struck out three in three shutout innings. Jesse Hahn worked three innings of two-hit relief and Frankie Montas went the final two innings of the five-hit shutout. Third baseman Ryon Healy, getting a start at first, homered.

Orlando Oberto started for Italy and worked two innings and gave up a run on two hits.

ANGELS 9, REDS 0

Ricky Nolasco went three innings, starting off a combined three-hitter. Mike Trout hit an RBI triple.

Amir Garrett gave up five runs, none of them earned, on seven hits in 2 2/3 innings.

BREWERS (ss) 7, DODGERS 2

Dodgers starter Rich Hill allowed three runs and walked four in 1 2/3 innings. Joc Pederson homered for Los Angeles.

Milwaukee reliever Neftali Feliz struck out two in a hitless inning.

BREWERS (ss) 5, WHITE SOX 4

Milwaukee newcomer Eric Thames doubled to raise his spring batting average to .167. Lewis Brinson, Travis Shaw and Ivan DeJesus Jr. each homered for the Brewers.

Avisail Garcia, Tyler Saladino and Rymer Liriano homered for the White Sox.

VENEZUELA 11, ROYALS 0

Miguel Cabrera hit a solo homer and a two-run single, Alcides Escobar tripled twice and drove in three runs against his big league team and Kansas City teammate Salvador Perez had two hits for Venezuela.

Royals starter Ian Kennedy gave up one earned run in two innings.

GIANTS 6, PUERTO RICO 5

Carlos Beltran and Angel Pagan each finished with multihit games for Puerto Rico.

Mike Morse hit a two-run homer for San Francisco, and Jeff Samardzija allowed one run through 3 2/3 innings. Chris Marrero, a former first-round pick of Washington in 2006, hit a game-ending, two-run homer.

DIAMONDBACKS 10, MEXICO 4

Arizona ace Zack Greinke struggled, allowing two runs and six hits in 2 2/3 innings.

Luis Alfonso Cruz had an RBI double and scored a run for Mexico and Efren Navarro had an RBI single.

TWINS 3, TEAM USA 2

Nick Gordon left off the sixth inning with a double and scored on a Byung Ho Park sacrifice fly to right field. The Twins got two-out RBI singles from Jason Castro and Jorge Polance off Andrew Miller, who relieved USA starter Luke Gregerson.

Team USA started the game with singles from Ian Kinsler and Adam Jones and a double from Nolan Arenado off the top of the fence off Twins starter Trevor May, who is trying to bounce back after his season was cut short by a stress fracture in his back. That was all he allowed in 3 2/3 innings. Eric Hosmer had a pair of singles

The play of the game was Byron Buxton hauling in Jones’ long drive to center before he smashed into the wall.

COLOMBIA 9, RAYS 7

Tim Beckman hit a two-run home run and doubled, and Steven Souza Jr. homered for Tampa Bay. Rays starter Chase Whitley gave up one earned run in three innings.

Tito Polo of the New York Yankees and Jorge Alfaro of the Philadelphia Phillies hit home runs for Colombia. Tampa Bay pitchers Jeff Ames, Neil Wagner and Diego Moreno combined for one hit in four scoreless innings against their teammates.

INDIANS 14, MARINERS 6

Adam Moore hit a grand slam, and Jose Ramirez had a three-run homer for the Indians.

Edwin Encarnacion hit a two-run double and 36-year-old catcher Erik Kratz also homered. Cleveland starting pitcher Carlos Carrasco pitched three scoreless innings.

Third baseman Kyle Seager had two hits and doubled for Seattle. Hisashi Iwakuma pitched two scoreless innings in his camp debut.