NEW YORK — Thousands of fans donned Mets caps with brunette locks flowing out the back at Citi Field on Saturday night — a promotional giveaway by the defending NL champs to honor follicly gifted right-hander Jacob deGrom.

Those lookalikes are probably the closest the Mets will see to deGrom pitching as they push for a postseason return.

DeGrom was scratched from his scheduled start Sunday and is unlikely to throw again this season because of an injury to the ulnar nerve in his right elbow.

The 2014 NL Rookie of the Year has been out since Sept. 1 after experiencing elbow pain in a start against Miami. DeGrom felt fine while throwing a bullpen Friday, but then sensed a twinge while tossing a baseball in from the outfield during batting practice about 15 minutes later.

“Just trying to lob it into the bucket,” deGrom said. “I guess throwing that bullpen had messed with that nerve. After I threw it, I was like, ‘OK, I have to go say something.'”

General manager Sandy Alderson announced deGrom’s prognosis Saturday. Hours later, the Mets gave away the hairy hats before a game against the Twins while hoping to stretch a two-game lead over St. Louis for the second NL wild card.

DeGrom will need surgery but has not decided whether he will wait until the end of the season to have it. Recovery time for the surgery is expected to be three months, meaning the 28-year-old will be ready for spring training.

DeGrom is 7-8 with a 3.04 ERA this season, including 0-3 with a 9.82 ERA in his final three starts. He said he began experiencing numbness in his right pinky and ring fingers about five or six starts ago and did not tell the team. He then felt pain in the game Sept. 1 and at that point notified the Mets.

“After the last one, I was like, ‘OK, something’s not right and I think we need to get this looked at,'” deGrom said.

An MRI showed no damage to deGrom’s ulnar collateral ligament — good news for the star righty, who already had Tommy John surgery in 2010. DeGrom said doctors told him it wasn’t unusual for Tommy John veterans to experience nerve issues caused by scar tissue buildup around the elbow.

Even before the flare up Friday, deGrom expected to have surgery this offseason to address the scar tissue.

“I think this is good news compared to what it could have been,” deGrom said.

The Mets will have to defend their wild-card lead without another pivotal piece. DeGrom joins Matt Harvey, David Wright, Neil Walker and Zack Wheeler as Mets likely done for 2016.

On top of that, ace Noah Syndergaard and lefty Steven Matz may require operations in the offseason to remove bone spurs in their pitching elbows. If that happens, New York could enter 2017 with five starters coming off surgery.

There was some good news Saturday. Slugging first baseman Lucas Duda was activated from the disabled list after missing nearly four months recovering from a stress fracture in his lower back. His role could be limited while he shakes off rust, and Duda said he hopes to contribute fully by the start of the postseason.

Matz meanwhile threw a 30-pitch bullpen, signifying he could return soon after already missing a month with left shoulder tightness. Manager Terry Collins said Matz will be evaluated Sunday before determining his next course of action.

The rotation is left with regulars Syndergaard and Bartolo Colon backed by rookies Seth Lugo and Robert Gsellman — Lugo is 4-1 with a 2.27 ERA in five starts, and Gsellman is 1-1 with a 3.57 ERA in four starts.

Gabriel Ynoa will start Sunday against Minnesota, and Collins said he expects to “use a plethora of people” to cover innings.

“You have to be resilient,” Collins said. “There’s no guarantees anything’s going to happen on a daily basis. So you have to get ready for tomorrow and hopefully the guys who go out there get some big outs.”