SAN DIEGO — Jay Bruce has broken out his first baseman’s mitt lately just to ensure it doesn’t turn to dust.

And he soon will need it for the Mets.

With veteran Adrian Gonzalez off to a sluggish start and hot-hitting Brandon Nimmo stuck without an opportunity for at-bats, Bruce is expected to receive at least one start at first base on the team’s next homestand, a club source told The Post on Saturday. The move will create another level of flexibility for manager Mickey Callaway, allowing Nimmo to become more involved.

Bruce’s latest prep work came Friday at Petco Park, where he took pregame grounders at first base. Bruce, who was 1-for-4 in Saturday’s 12-2 loss to the Padres, said he also took grounders at first base before another recent game.

“I think they want me to do it honestly when we’ve been on the road and there have been some really nice surfaces,” Bruce said. “They are like, ‘Hey, take some grounders.’ My philosophy on it is still the same. If they ask me to do it, I will do it. If they need me to do it, that’s fine. But until they tell me otherwise, I am going to just keep going about my business and I will do whatever they ask me to do.”

The 35-year-old Gonzalez took a .203/.300/.322 slash line with two homers and 12 RBIs into Saturday’s action. Gonzalez has played as part of a platoon with Wilmer Flores, who starts against most lefties.

But with a crowded outfield, the Mets will look to utilize Nimmo, who has been among the team’s early bright spots, slashing .296/.500/.593 with one homer and two RBIs in 27 at-bats entering play.

Nimmo’s rare chances to start have come at the expense of Bruce and Michael Conforto, but Juan Lagares sees action in center against lefties, further complicating the equation for Callaway.

So Bruce playing an occasional game at first will help dissolve the logjam.

“I don’t make those decisions, but looking at it on a piece of paper, it makes sense,” Bruce said. “It’s a way to keep all of our bats in the lineup, but we’ll see. They have a lot of stuff they have to do as far as decisions they have to make.”

Bruce appeared in 11 games at first base last season for the Mets, but at this point doesn’t view himself as an option to play the position on more than a limited basis.

“If I was going to make the full-time switch there at some point it would be an offseason thing,” Bruce said. “All offseason and spring training working toward it, just to feel like I knew the ins and outs, but when I do go over there I just try to feel real natural.”

To that end, Bruce has told infield coach Gary DiSarcina to avoid presenting him with too much information too soon.

“I feel pretty comfortable when I am out there, so unless he sees something that is not going to work and I cannot play the position doing that I just want to ‘sandlot’ it and just play,” Bruce said. “But everything has been good as far as my hands and stuff and if it was a more permanent situation I would focus much more on footwork, but it feels OK. I don’t think there is any craziness.”