If a movie were to be made about the San Francisco 49ers' season, quarterback Brock Purdy said it would be called You Just Never Know.

That title would also describe Purdy's Cinderella career, in which he has gone from NFL's 'Mr Irrelevant' to 'Mr Irreplaceable' in two short years, leading the 49ers to Sunday's Super Bowl (Monday AEDT) against the Kansas City Chiefs.

As the last player selected in the 2022 draft held in Las Vegas, Purdy earned the dubious distinction of Mr Irrelevant - a brief moment in the spotlight before slipping into the shadows like many final picks before him.

Often those players do not even make it as far as training camp, released by the team that drafted them long before the season starts.

San Francisco coach Kyle Shanahan saw something unique in Purdy, though, and kept him with the team - albeit well down the pecking order behind starter Trey Lance, the third player selected in the 2021 draft, and veteran Jimmy Garoppolo.

But last season, when Garoppolo and Lance both went down with injuries, the rookie stepped in and seized his chance to take the starter's role.

He never let go.

"It starts with being able to believe in yourself," Purdy said during Super Bowl Opening Night on Monday.

"People are going tell you that you may not be good enough ... but if you believe and you keep putting in the work then anything is possible."

It seems the stars are aligning for Purdy.

While Shanahan rolled the dice and hit the jackpot, it is doubtful anyone else bet on the 24-year-old Purdy returning to Las Vegas as a first string quarterback.

"It's just cool how it all is in a way full circle," Purdy said.

"I got drafted here last and now here we are two years later to get ready to play in the Super Bowl."

In helping San Francisco get back to the Super Bowl and a chance to win their sixth Lombardi Trophy - and a first since January 1995 - Purdy put himself into the 'most valuable player' discussion directing the 49ers' high-powered attack.

Purdy enjoyed a solid but unspectacular college career, and it is his unflappable, humble demeanour, clinical skills and high football IQ that have made him an NFL success story.

"I love Brock," said Shanahan.

"I have so much respect for Brock as a player, as a person.

"Just to watch someone like Brock do what he's done, the way he is. He got undersold.

"You can't be great in this league unless you don't have like 20 things; Brock's got it all."