PORT ST. LUCIE — Paul Blackburn’s participation in normal physical activity late last season left him feeling as if his spine was getting crushed.

Running, jumping, walking up and down stairs were activities that left him in severe discomfort.

Migraines persisted, along with tingling sensations on the left side of his body.

The Mets right-hander couldn’t find an answer for his problems.

Finally it was discovered Blackburn’s issues were caused by a cerebrospinal fluid leak.

The fix was October surgery in which doctors shaved bone spurs around a capsule in the spine that was punctured.

The spinal fluid was drained during the procedure and the capsule stitched.

Blackburn spent the winter at home in Idaho resting for much of the offseason before picking up a baseball in mid-December.

He might be a tad behind schedule this spring, but he’s clearly in the Mets’ rotation mix.

Blackburn’s build-up will be more closely scrutinized after Frankie Montas was shut down in recent days with a high-grade lat strain.

Montas won’t be an option to begin the season with the club — Mets officials say he won’t throw for at least 6-8 weeks — leaving Blackburn and Tylor Megill as the top possibilities for that spot.

The fact Blackburn is without minor league options (Megill can be optioned to the minors) places him as the front-runner for the spot provided he is healthy.

“I expect to be ready to go,” Blackburn said.

Tuesday he faced hitters for the first time this spring in one inning of live batting practice.

“I like the sinker. That is a pitch he is working on,” manager Carlos Mendoza said. “The fact that he is facing hitters after what he went through in the offseason, that is a big step in the right direction.”

Brandon Nimmo said his focus this week is “upping the load” on his left foot to more than sprint speed as he participates in full-squad workouts. The outfielder, who is recovering from plantar fasciitis in the foot, said last week he was running at about 91 percent of his top foot speed. This week it’s more about “being in cleats the full day” and “base-running drills,” according to Nimmo.

Sean Reid-Foley apologized multiple times to Francisco Lindor after buzzing the Mets shortstop with a high fastball during live batting practice.