On a play-action pass, Chargers running back Najee Harris crumpled to the turf before the fake handoff could fully develop, immediately grabbing his left ankle and tossing aside his helmet in pain.

Needing assistance, trainers helped Harris to the sideline, as he was unable to put any weight on his leg, before he was carted to the locker room in the second quarter of a 23-20 win over the Denver Broncos at SoFi Stadium on Sunday.

Harris, who spent the lead-up to his first season in L.A. recovering from an offseason eye injury in a fireworks accident, was expected to be a key piece of a one-two punch with rookie Omarion Hampton.

Now, he appears to be sidelined for the season with an Achilles injury, according to head coach Jim Harbaugh, who called the diagnosis “preliminary” as Harris underwent postgame imaging.

“Not good,” Harbaugh said of his emotions as the play unfolded. “[I was] just hoping for the best — maybe a high ankle, something else that wouldn’t be long-term.”

Speaking at the podium with a somber tone, Harbaugh said he met with Harris at halftime and described the running back’s demeanor as “cold-blooded,” adding that he told him: “You’ll be back, kid.”

The injury appeared clear on film, according to Dr. Dan Ginader, physical therapist and author of “The Pain-Free Body,” who reviewed video of the play.

“When looking at the calf of the back plant leg, you can see the muscle sort of ‘jump’ which is indicative of a complete tear of the Achilles,” Ginader said. “Players who have suffered this injury often describe it as being hit in the heel with a shovel. … When you see the muscle jump and see the player crumble to the floor, you can be pretty sure it’s a complete tear.”

Before going down, Harris had been featured early Sunday, carrying six times for 28 yards. Durable throughout his career, he had appeared in all 71 games across five NFL seasons before the injury.

If it’s a complete tear, the earliest Harris could return is about eight months, Ginader said, though most players don’t feel fully themselves “until at least 12 months” post-surgery. For a skill player, he added, “it takes longer to be able to come back at full force.”

With Harris out, Hampton is expected to shoulder a bigger role moving forward. Hampton, who calls Harris a mentor, admitted the loss stings.

“It definitely hurts,” Hampton said.