Mets prospect Peter Alonso celebrates after hitting a two-run homer...

Mets prospect Peter Alonso celebrates after hitting a two-run homer in the All-Star Futures Game on July 15, in Washington. Credit: Getty Images/Rob Carr

MIAMI — Mickey Callaway is hoping to see first-base prospect Peter Alonso, the first player in the majors or minors to reach 100 RBIs this season, with the Mets in September, the manager said Friday.

But it’s not clear that he’ll get what he wants.

“He’s an interesting kid,” Callaway said. “He’s a hard worker, we found that out in spring training. He’s willing to do everything he can to improve every day.

“He’s getting to where we want him to get [defensively]. Obviously still has some things to learn because he’s a young player, but he’s definitely improving.”

Assistant general manager John Ricco said recently the Mets want to leave Alonso with Triple-A Las Vegas through the minor-league season, which ends Sept. 3, and that they haven’t discussed a potential September call-up.

One issue would be playing time. Wilmer Flores has been the Mets’ starter at first, and Jay Bruce (right hip strain) is expected to see time there upon his return from the disabled list. The Mets also say they plan to bring first baseman/outfielder Dominic Smith back to the majors at some point.

The knock on Alonso, 23, is his defense. The Mets say he’s improving, and in July he was Las Vegas’ defensive player of the month, an in-house honor the organization uses to reward a player for his hard work. External reports aren’t as kind — questioning the ability, not the work ethic — and last fall Alonso attended the Mets’ instructional league for extra reps at first, an unusual step for a player at his age and stage.

Yet it’s hard to deny the potency at the plate. Another multihit game Thursday upped Alonso’s slash line to .251/.350/.556, a vast improvement over his transition-period struggles after being promoted in mid-June. In Las Vegas and Double-A Binghamton combined, Alonso has 28 homers and 103 RBIs after a two-run single late Friday night.

“He’s definitely improving defensively,” Callaway said. “And obviously we’ve all seen what he’s been doing at the plate, which has been pretty impressive all year.”

Rotation rumblings

Steven Matz (left flexor pronator strain) is scheduled for a simulated game Saturday, Callaway said. That would put him in line to pitch in the doubleheader Thursday against the Phillies if the Mets deem him ready.

Upon Matz’s return, the Mets are considering a six-man rotation, Callaway said. That would allow them to keep starting Corey Oswalt, who has been pitching well and goes again Saturday, and Jason Vargas, who has had a bad season but to whom the Mets seem to be committed.

“That would be something we’re definitely considering,” Callaway said. “We’ve talked about it a couple of times, especially since we want to monitor guys like [Noah] Syndergaard and [Zack] Wheeler, a six-man rotation might make sense at some point.”

Rest assured, the Mets would take Jacob deGrom’s pursuit of the NL Cy Young Award into consideration if they do move to a six-man rotation.

“We would have a conversation with Jake,” Callaway said. “We understand where he’s at and what he’s trying to do. We would make sure that he’s taken care of.”

Extra bases

Bruce will go on a minor-league rehabilitation assignment “very soon,” said Callaway, who did not specify a day. He has been out since June 18 ... Infielder T.J. Rivera doesn’t sound particularly close to starting another rehab assignment. He was diagnosed last month with a strain of his elbow, already repaired by Tommy John surgery last September. “They’re still trying to make sure everything is good,” Callaway said.

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