KANSAS CITY, Mo. — Brandon Nimmo’s nearly three-month stay on the injured list is closer than ever to ending.

Nimmo began a rehabilitation assignment Friday, batting second as the DH for advanced Class A St. Lucie, a positive development for Nimmo and for a Mets team that could really use the lefthanded hitter’s bat. He went 1-for-3 with a walk and a strikeout in a 7-0 loss at Charlotte.

Manager Mickey Callaway said the team doesn’t know how long Nimmo will need to play in the minors before returning. But considering he has been out since May 21 with a bulging disc in his neck, it likely won’t be a short assignment.

“We’re excited to see how that goes,” Callaway said. “It’ll be a situation where we monitor him daily and figure out what the schedule is, but we’re optimistic he’s going to do fine. We just have to see how long it’s going to take for him to get back.”

After his breakout 2018, this has been a lost season for Nimmo, who has a .200/.344/.323 slash line in 43 games. He was doing OK before hurting his neck in mid-April, tried to play through it for a month, went on the IL and tried a rehab assignment in early June, but the neck continued to bother him. That’s when the Mets sent Nimmo to a specialist and learned the extent of the issue.

The Mets, meanwhile, are without the lefthanded-hitting Robinson Cano (torn left hamstring) indefinitely and Jeff McNeil (strained left hamstring) for at least another week.

“We’re kind of righthanded-dominant at this point. To get another lefty would be big for us,” Callaway said. “Obviously, we have some guys down. To get some guys back would be a huge boost for us.”

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Callaway said he hasn’t had to talk with Nimmo — known for being smiley and happy — about keeping his spirits up. Quite the opposite, in fact.

“Keep Nimmo’s spirits up?” Callaway said. “I talk to him to keep my spirits up. I just look up pictures on the internet to make myself smile.”

Sewald returns

The latest roster churn involved a trio of righthanded relievers Friday: Paul Sewald up from Triple-A Syracuse, Drew Gagnon down to Syracuse and Tim Peterson designated for assignment (to make room for Sewald returning to the 40-man roster).

Gagnon allowed five runs (four homers) in 1 2⁄3 innings against the Braves on Thursday night, turning a blowout into a nail-biter.

Sewald has a 2.08 ERA (three earned runs in 13 innings) since the All-Star break for Syracuse.

“Slider has been a pitch for [Sewald] that has really improved over the course of his time in Triple-A. He’s been working really hard on it,” Callaway said. “We feel like he can get the job done.”

Injury updates

J.D. Davis (tight right calf) was removed in the eighth inning of the Mets’ 4-1 loss to the Royals on Friday. Davis called it precautionary and said he doesn’t expect to miss any games ... Robert Gsellman (sore right triceps the past few days) was unavailable, Callaway said.

Amed & Didi

Callaway said there are similarities between Amed Rosario and Yankees shortstop Didi Gregorius, whom Callaway saw a bunch when Gregorius was a Reds minor-leaguer sharing a complex with Callaway’s Indians.

“You didn’t quite know how good [Gregorius] could be, but you saw the potential,” Callaway said. “And then his first couple years in the big leagues, he was solid. But then it was a whole different level.

“Rosie is really starting to show the power, the ability to hit for average, the defense. It’s all coming along.”

Extra bases

A new name to watch on the August waiver wire: outfielder Billy Hamilton, designated for assignment Friday by the Royals. If and when Hamilton becomes a free agent, his all-glove, no-bat skill set could be an upgrade for the Mets, who have Aaron Altherr as their reserve outfielder. . . Wilson Ramos was out of the lineup Friday for the first time since Aug. 5, though he had a pinch-hit single in the ninth inning. Callaway called it “a much-needed day” off after nine consecutive starts behind the plate.