SANTA CLARA >> Practicing behind a patchwork line is not how Brock Purdy dreamed of spending his 25th birthday Friday, never mind that he just gifted 49ers linemen a caravan of trucks and sports-utility vehicles on two-year leases.

The 49ers are poised to field their most inexperienced offensive line in at least coach Kyle Shanahan’s eight seasons.

Monday night’s chore for whoever blocks: protect Purdy and clear lanes for ballcarriers Isaac Guerendo and Deebo Samuel against the NFC-leading Detroit Lions at Levi’s Stadium.

Injuries have so thoroughly depleted the Niners (6-9) that three of five spots could feature linemen making their 49ers starting debuts. The only healthy starters are center Jake Brendel and right guard Dominick Puni. Offensive line coach Chris Foerster plans on keeping that duo in their regular roles while “plugging in three guys around them.”

“More than anything, it’s communicating to them about the cadence and timing of plays, just helping them in that regard, and talking to them at lunch,” Purdy said. “It’s new, it’s different, it’s a tough situation, but this is the NFL so we have to adapt and be ready to go.”

When Foerster entered his news conference after Friday’s practice, he quipped to reporters: “If there’s five of you, we can have a tryout right now. ... It’s a challenge for me but a great opportunity for guys entering the lineup to show what they can do.”

Left tackle Trent Williams and his backup, Jaylon More, both went this week on injured reserve, which is where left guard Aaron Banks (knee) is expected to land before Monday night’s 5:15 p.m. kickoff. Right tackle Colton McKivitz did not practice Thursday after being limited by a knee issue, and versatile backup Spencer Burford remains sidelined with a calf injury from subbing at left tackle in Sunday’s loss at Miami. Offensive linemen Ben Bartch and Jon Feliciano also are on injured reserve.

Nick Zakelj is expected to make the first start of his career, most likely at left guard. “I know it’s just an amazing opportunity to have,” Zakelj said. “I can’t wait to go grasp it and go out there, fly off the ball and let it loose.”

Charlie Heck, poached Wednesday off the Arizona Cardinals’ practice squad, made 21 career starts for the Houston Texans from 2020-23, and Thursday acquisition Matt Hennessy made 22 starts as the Atlanta Falcons’ 2020-22 center. No other linemen have starting experience, including Zakelj, Austen Pleasants, Sebastian Gutierrez, Isaac Alarcon and undrafted rookie center Drake Nugent.

“When it comes to the pass game, I can do my part getting the ball out, being on time and help them establish their feet within the game,” Purdy said.

Said tight end George Kittle: “When you lose offensive linemen, it limits you to certain things you can and can’t do. But our game plan is fun. There are a lot of things we haven’t done for a while that we’ll be able to pull out.”

Noting he felt before the season the 49ers had nine starting-caliber linemen, Foerster said: “That was good depth to have.”