Long-ball king Pete Alonso is now also a master of the short recovery.

The Mets reinstated Alonso from the injured list Sunday and sent top prospect Mark Vientos to Triple-A to clear a roster spot before losing 8-7 in the series finale against the Cardinals at Citi Field.

Originally expected to be sidelined three to four weeks with a bone bruise and sprain in his left wrist, the National League’s home run leader only missed 10 days — the minimum stay required on the injured list.

“Right around the fifth or sixth day, I was able to walk around freely and without any pain,” Alonso said before going 0-for-4 in the cleanup spot and booting a hard grounder for an error at first base. “If I’m at 100 percent, why wait?”

Alonso declared himself “full go” without any limitations and said doctors were erring on the cautionary side with that original recovery timetable.

Manager Buck Showalter said the Mets were “lucky so far” that Alonso was able to return so quickly without any setbacks.

He was initially injured when Braves starter Charlie Morton hit him with a pitch on June 7.

“He felt fine physically,” manager Buck Showalter said, acknowledging he hadn’t yet received a final report from trainers. “He was irritable about how the game went today. He was looking forward to getting back and making a contribution.”

The Mets (33-37) went 3-5 without Alonso and slipped to 11 ¹/₂ games behind the National League East-leading Braves.

Shohei Ohtani (23) passed Alonso (22) for the MLB lead in home runs but Alonso still is ahead of Jorge Soler, Matt Olson and Kyle Schwarber (20 apiece) for the NL lead.

Alonso trained at the ballpark during both team off days last week, swung in the batting cage and fielded grounders before Saturday’s game.

“I didn’t have any residual swelling after I did pretty heavy workloads,” Alonso said. “I felt good consistently and I’m happy we’re in a spot where I can play. There was a big question mark the day I got hit and then especially when I needed to go get additional imaging, but the biggest point in this is my body responded well to the treatment.”

Vientos crushed Triple-A pitching — 13 home runs and a 1.104 OPS in 38 games earlier this season — but struggled to find his place in the majors.

He hit one home run — in his first game — and managed a .469 OPS in 16 games, and Alonso’s return to first base was about to further limit Vientos’ already sporadic at-bats.

“I didn’t want to necessarily push anything to the point where I’m harming myself,” Alonso said, “but if my body kept progressing in a positive way then I wanted to keep hitting those benchmarks as quickly as I could so I could get back as quickly as I could. I take pride in being out there and playing as many games as possible.”

Alonso wasn’t about to second-guess the Mets’ decision to put him on the injured list rather than have him eligible to return a few days earlier.

“I wish I could’ve played [Friday and Saturday], but if I was to stay active on the roster that would be pretty selfish because I would be taking up a spot from somebody,” Alonso said. “I think taking the full 10 days was necessary for this. I’m really stoked to be back.”