Slowly but surely, the Mets’ injured list is beginning to return some of its firepower to the active roster.

Pete Alonso is expected to be activated off the injured list Monday, when the Mets begin a series against the Diamondbacks in Arizona, with reliever Seth Lugo and outfielder Kevin Pillar possibly joining him.

Alonso’s arrival will also force manager Luis Rojas to make a hard decision on his catchers’ playing time, after both had been starting with James McCann filling in for Alonso at first base, but a thriving Tomas Nido isn’t going away behind the plate.

Either way, a healthy Alonso would deliver a big jolt to a lineup that was down to three regulars without him.

“Huge presence,” Rojas said Sunday after the Mets and Braves were postponed at Citi Field. “[James] McCann has done really good as a first baseman for us lately, but to have Pete in there, the impact of his presence, it changes a lot of things.”

Alonso went on the 10-day IL on May 21 with a right hand sprain that stemmed from getting hit by a pitch on May 5 in St. Louis — at which point he was hitting .287 with five home runs and a .892 OPS. He initially tried to play through it, but since the game in which he was hit, he batted just 5-for-40 (.125) with one home run and a .505 OPS in 12 games before finally landing on the IL.

In Alonso’s absence, the Mets held down the fort to go 5-3. They also shifted McCann to first base to fill in for Alonso, starting the catcher there in four of their last five games. He handled himself well defensively and came alive Saturday with a four-hit night.

Just before Alonso was sidelined, the Mets had begun to give Nido more starts behind the plate. He has hit 10-for-31 over his last nine games, ensuring he will still be plenty involved even without the option of having both catchers in the lineup.

“[McCann] and Nido, as of now, are going to split the catching time,” Rojas said. “Mac may get his time at first as well, since he’s shown us he can do it. Having [Patrick] Mazeika on the roster helps us have some moving parts there and have both of the catchers if we need them to be in the game. But as of now from the catching position standpoint, those guys are going to split time.

“Nido’s done a really good job catching and hitting and McCann’s coming up — he’s also caught very well, but he’s coming up with his bat as well.”

The Mets put on a power show Saturday night with an unlikely group — McCann, Jonathan Villar, Brandon Drury, Billy McKinney and Francisco Lindor — crushing five home runs in a 13-2 win over the Braves. But having Alonso back in the heart of the lineup provides the Mets a more dependable threat.

“His presence is definitely something we need,” Rojas said. “If we hit for power, which is something Pete can do, we can score a lot of runs. We had a sample of that [Saturday]. So having Pete here, our most powerful presence in the lineup, it can lead to that.”

The Mets’ IL at one point hit a high of 17 players, but after Taijuan Walker got activated Saturday and returned to dominant form on the mound, Alonso, Lugo and Pillar would cut that number to 13 — still with significant players sidelined, but an important step in the right direction.

Lugo, who underwent surgery in February to remove a loose body from his right elbow, threw 4 ¹/₃ innings across four games on his rehab assignment.

Pillar, meanwhile, defied expectations with his rapid return. He was hit in the face with a 94.5 mph fastball on May 17, suffering nasal fractures that required surgery last Monday. But he quickly advanced to baseball activities and, like Alonso, has looked good hitting off the high-velocity pitching machine, according to Rojas.