Paul Blackburn’s last appearance in a regular-season major league game was against the Padres last season, where he struggled and was forced to leave the game after being hit by a line drive in his right hand.

He’s set to return to a major league mound Monday when the Mets open a series in Los Angeles against the Dodgers.

Following that game, the 31-year-old pitcher experienced back discomfort that led to him being placed on the injured list after a rehab outing in September. It was during this time that a surprising discovery was made – a spinal fluid leak in his back.

After undergoing a series of tests and seeking multiple medical opinions, Blackburn was informed by specialists that fixing the issue would require inserting a pin or a hinge in his spine. Unfortunately, this procedure would likely mark the end of his playing career.

But he ended up having bone spurs shaved around a capsule in the spine in the area that was punctured.

The October procedure made it possible for him to return.

“People I went to said they’d never seen this in a baseball player and they didn’t know how I’d come back from it,” Blackburn said Saturday before the Mets defeated the Rockies, 8-2, at Citi Field. “I wasn’t sure if I’d be able to pitch again. That [thought] was there. That was real. So when I went through the rehab process, I tried to be as positive as I could and focus on the day.”

The plan worked well enough that Blackburn was able to resume throwing in the offseason and arrived to spring training feeling good.

He said he began to feel like himself while throwing “probably in mid-January.

For a couple of months after the surgery, there were times I would ache and it would be uncomfortable.”

Blackburn said he now has more movement in his back and “feels normal.”

His return was delayed in the spring by right knee inflammation, but he’s since made seven minor league rehab starts, including the last two at Triple-A Syracuse, where he pitched well.

“There’s nothing lingering,’’ Blackburn said.

For a pitcher who’s entering his ninth major league season, Blackburn said there may be more emotion attached to this start than his usual first start of the year.

“I’m definitely excited,’’ said Blackburn, who arrived in a trade from the A’s last July in exchange for minor league right-hander Kade Morris.

“It feels like it’s been a long time coming. When you make your first start of the year in the big leagues, it brings a lot with it. There’s a different sense of adrenaline, anxiety, nerves, all of it.”

And that’s especially true now, when Blackburn said he was aware it might not happen again.

“It’s been a long time coming,’’ Blackburn said.

Blackburn’s return comes with the Mets expanding to a six-man rotation at least one time through the rotation, as they are in the middle of a stretch of playing 10 games without a day off.

President of baseball operations David Stearns said Friday the team would likely fluctuate between a five- and six-man rotation for the foreseeable future, depending on the schedule and the needs of individual pitchers.

For now, though, Blackburn has a spot.