It was a day of “firsts” for rookie Matt Reynolds.

Reynolds — an infielder since college — can check off his first outfield start and not only his first home run, but first game-winning homer off the proverbial baseball bucket list. His sixth-inning solo home run to right-center field gave the Mets a 4-3 lead and they held on to win by that score to complete a two-game home sweep over the Royals Wednesday afternoon.

“It was probably one of the biggest moments I’ve had in my career as a baseball player, not just including professionally,” Reynolds said after his 18th major league game. “It was a great feeling just to help the team win.”

Despite never playing a professional game in the outfield, Reynolds appeared ready for the task. He made the routine plays and delivered a throw that nearly nailed Salvador Perez after bare-handing a ball off the wall and two-hopping a hard throw into second.

“I was a little surprised but I believe in my ability and I believe that I can play almost any position on the field,” said Reynolds, who said he used Juan Lagares’ glove. “I didn’t really have too many plays that tested me today but I felt very comfortable.”

Manager Terry Collins wanted to give Reynolds opportunities to play, and even had the 25-year-old take fly balls during spring training in case he would be needed in the outfield. Collins said he thought Reynolds’ athleticism would help him succeed in multiple positions.

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Reynolds was removed after the sixth inning in a double switch to insert Michael Conforto and lift Noah Syndergaard, who left after 91 pitches due to “minor elbow discomfort.”

The Mets took a 1-0 lead in the fourth on James Loney’s two-strike RBI single to left, driving in Asdrubal Cabrera, who made an athletic slide to get around the throw that ran up the third-base line.

The Royals answered, taking a 2-1 lead in the fifth on Cheslor Cuthbert’s solo home run followed by Whit Merrifield’s RBI single three batters later. It didn’t take the Mets long to respond as Cabrera sent a 96-miles per hour fastball over the left-center field fence in the bottom half of the inning to regain the 3-2 advantage for the shortstop’s second home run in as many games.

But, again, Syndergaard failed to hold the lead. Perez doubled and scored in the sixth on Paulo Orlando’s single to tie the game at 3. Reynold’s heroics emerged in the bottom half of the frame and the combination of Jerry Blevins, Addison Reed and Jeurys Familia tossed three innings of scoreless relief. Familia recorded his major-league leading 24th save.

Five of the Mets’ six runs in the sweep came courtesy of homers by four different hitters. Recent injuries haven’t made Collins’ job any easier, but it’s given new players openings to prosper.

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“All I know is guys grab opportunities and run with them,” Collins said. “Some guys take those opportunities and get the most out of them. Right now, we got some guys who are going to help out and we need them to step up and today somebody did.”