Brock Purdy is penning some kind of story. 

“Mr. Irrelevant’’ from the 2022 NFL Draft — the nickname given to the last player selected — is anything but irrelevant entering the NFL playoffs. 

The 23-year-old rookie quarterback for the 49ers is one of the most remarkable stories to this NFL season and he writes the latest chapter to his improbable tome on Saturday when 13-4 San Francisco plays 9-8 Seattle in the NFC wild-card playoff game at Levi’s Stadium. 

Raise your hand if you fired up your betting app before the season and put money on an NFC playoff game between the 49ers and Seahawks pitting Brock Purdy and Geno Smith as quarterbacks. 

Purdy is the third quarterback the 49ers have played this season in what has become one of the most bizarre quarterback-carousel roller coasters in memory. 

Trey Lance, head coach Kyle Shanahan’s choice as the franchise quarterback, started the first two games and was lost for the season with a broken leg in Week 2. 

Enter Jimmy Garoppolo, the veteran who despite leading the team to a Super Bowl three years ago was seemingly unwanted by the franchise, which grudgingly brought him back for 2022 as an insurance backup. Garoppolo played well, perhaps saved the team’s season going 7-3 as the starter, until he suffered a broken foot on Dec. 4. 

Enter Purdy, an afterthought third-stringer out of Iowa State. 

All he’s done is lead the 49ers to wins in their final five games of the regular season, completing 67.1 percent of his passes for 1,374 yards with 13 TDs to just four INTs and a 106.3 rating. 

Raise your hand if you placed a bet on the 49ers being a 9.5-point favorite to win a playoff game with Brock Purdy at quarterback. 

What a story. 

It’s a story that has drawn the attention of — and comparison to — a Hall of Fame quarterback who authored one of the greatest unlikely journeys in NFL history, Kurt Warner. 

The surface comparisons are simple. Warner is an Iowa native who played his college ball at Northern Iowa and, when he was ignored by the NFL, became a star for the Iowa Barnstormers in the Arena Football League before finally carving out his improbable NFL career that included a Super Bowl title. 

Warner, an NFL Network analyst who now lives in Arizona (where Purdy is from), has kept a close eye on Purdy’s journey and smiled. 

“It’s been fun to watch Brock’s journey,’’ Warner told The Post on Friday. “They overlooked me and they overlooked him to a certain degree, because he was picked so late [No. 262 overall].’’ 

Since the NFL-AFL merger in 1970, nine backup quarterbacks have led their teams to championships. Among that group are Hall of Famers such as Warner. 

While he’s thoroughly embracing Purdy’s journey, Warner wants everyone to “sit back and let him write his own story.’’ 

“Tom Brady when he won his first Super Bowl wasn’t the Tom Brady that we know now, the greatest of all time,’’ Warner said. “He had a good team, he threw for 150 yards [145] in the [first] Super Bowl and one TD in the playoffs that year. He was able to become Tom Brady. 

“We’ll know who Brock is four or five years from now. Right now, he doesn’t have to be the next Tom Brady, he doesn’t have to be the next Kurt Warner. Just go do your thing this year and embrace this moment. 

“Eventually, we’ll start to see, can Brock Purdy be a franchise quarterback, can Brock Purdy be an All-Pro, can Brock Purdy be a Hall of Famer? We’ll get to see that down the road. We don’t need to see that right now.’’ 

Right now, Purdy is simply the most remarkable success story in the NFL. And he has the moxie of someone who believes he belongs and the numbers to back it up. 

“It doesn’t look like it’s too big for him,’’ Warner said. “And that’s what excites you. If he’s doing this right now, maybe he’s got a shot, maybe he can be one of those guys.’’ 

When he takes the field Sunday, Purdy will be the lowest-drafted rookie quarterback to start a playoff game — by 110 picks. 

Since 2010, according to ESPN, rookie quarterbacks are 2-9 in playoff games. You think Purdy cares about that stat? Here’s one that matters more: The 49ers averaged 22.6 points before Purdy started and are averaging 33.5 points in his five starts. 

Perhaps more magic awaits. No rookie has ever led his team to a Super Bowl. 

With his career complete, his Super Bowl title and his Hall of Fame jacket hanging in the closet, Warner wrote a best-selling book about his journey that’s since been made into a movie. 

Purdy doesn’t have a publisher yet, but he’s writing away. With his mind, moxie and his right arm.