Buck Showalter had a hard time formulating a lineup Wednesday, with more bats that he believes warrant playing time than there was playing time to dish out.

Upon the Mets calling up Mark Vientos, Showalter inserted the hot-hitting top prospect into the lineup at third base and gave breathers to lefty-hitting Jeff McNeil and Brett Baty.

After Vientos responded with a game-tying home run in his season debut, the lineup will become increasingly difficult to fill out because Vientos might not let Showalter sit him on the bench.

Vientos drilled a seventh-inning, two-run dinger to center field hours after the Mets called him up in an 8-7, 10-inning win over the Rays at Citi Field.

“Feels good to get the first one out of the way,” Vientos said after going 1-for-4, including a homer against righty Ryan Thompson that tied the game in the seventh. “First hit, first home run. Now I can just go out there and play.”

At least, he hopes he can play. In summoning the slugging Vientos, whose bat earned the promotion, the Mets created an odd roster alignment with two players (Vientos and Daniel Vogelbach) who don’t have a true position.

But they can afford to sacrifice flexibility if Vientos continues to hit like this.

The 23-year-old had demolished Triple-A pitching, posting a 1.104 OPS with 13 home runs in 38 games, which was especially tantalizing for a Mets offensive attack that has struggled, particularly with power.

Vientos arrived in Queens with a poor defensive reputation as a corner infielder.

Baty has handled third and Pete Alonso is entrenched at first base. Vientos’ best bet for playing time will come at DH, where Vogelbach has hit well against righty pitchers.

How much will Vientos play?

“We’ll see. It’s kind of up to him,” Showalter said of Vientos, who made an early statement toward earning more playing time. “Play good and we’re always looking for ways to get good players in there.”

What is Vientos expecting?

“To be playing,” Vientos said before the game. “To be ready.”

Vientos debuted at the end of last season, when he primarily served as a DH or pinch hitter and virtually only played against lefties.

He earned his way to a debut as a big-time power bat but one who whiffed too often, striking out 28.6 percent of the time with Syracuse last year.

He entered the offseason with a goal of cutting down on that number and has become more selective at the plate. He struck out in 20.5 percent of his Triple-A plate appearances this season.

“Instead of chasing and swinging at pitches the pitchers want me to swing at, waiting for my pitch and not trying to miss it,” said Vientos, who hit .167 with one home run in 16 major league games last season. “I feel like I’m in a better spot, like I’m a lot better than I was last year.”

The Mets are open to a savior, and Vientos sure looked like one quickly. Their offense did not hit a home run in their entire weekend, four-game series split in Washington, and the attack had lacked a power bat like Vientos’.But where he finds time becomes a major question around the club.

Wednesday, he played third base against an opposing lefty (Josh Fleming) as Baty sat and switch-hitting Eduardo Escobar played second base. The Mets will see a righty Thursday (Taj Bradley), which normally would mean Baty plays third, McNeil plays second, either Mark Canha or Tommy Pham plays left field and Vogelbach serves as the DH.

If Vientos continues to perform, he could take away time from any of the above, apart from McNeil.

The win-now Mets likely will field lineups soon containing three highly touted rookies in Vientos, Baty and Francisco Alvarez. The three overlapped plenty going through the system and took batting practice together hours before the game.

Vientos and Alvarez then looked like they were taking BP with homers in the seventh and ninth, respectively.

“We’re basically brothers,” Vientos said of Alvarez and Baty. “It gives us that confidence that we go out there and we just do our thing, just like we were doing at Triple-A.”