MILWAUKEE – Francisco Alvarez will be on standby when the Mets arrive at Citi Field on Friday.

The 21-year-old catcher – the organization’s top prospect – will be a possibility for addition to the major league roster, according to a source, after Omar Narvaez left Wednesday’s game with discomfort in his left calf.

“I felt tightness,” Narvaez said. “I’d rather call it for a day and be safe for the next day. I couldn’t put myself at risk for the whole season. I wasn’t going to play the hero.”

Tomas Nido replaced Narvaez behind the plate in the bottom of the ninth inning in the Mets’ 7-6 loss to the Brewers.

Narvaez, who went 1-for-4 in the loss and is hitting .286 in the early going, indicated he is unsure if he will receive imaging on the calf.

Alvarez spent his spring working on his receiving skills and struggled offensively, but the Mets are confident in his ability to hit.

Alvarez is off to a hot start for Triple-A Syracuse, where he owns a 1.056 OPS with two homers in four games. He hit a two-run blast on Wednesday in Syracuse’s 11-4 loss to the Rochester Red Wings.

Last year the Mets called up Alvarez for the final week of the regular season and watched him hit his first major league homer.

Alvarez was added to the NL wildcard roster, and went 0-for-1 in the series.

He hit just .107 (3-for-28) without a homer in spring training during Grapefruit League play, however, and took his demotion in stride when told about it before the start of the regular season.

“The first thing that came to mind [Wednesday] was I need to go back down there and work hard and prove myself so they can call me back up eventually,” Alvarez said through an interpreter.

The Amazins’ homestand begins Friday after the team announced the scheduled Thursday opener was postponed due to expected inclement. weather.

The game against the Miami Marlins will be made up on Friday at 1:10 p.m.