Rams backup quarterback Sean Mannion ready to show his stuff in first start

Though he never started a regular-season game for the Rams, quarterback Sean Mannion spent nearly three full NFL seasons preparing as if he was going to do just that.

On Sunday against the San Francisco 49ers, Mannion finally gets his opportunity.

With a playoff game coming up next week, quarterback Jared Goff is among the players that coach Sean McVay has decided to rest and keep out of harm’s way. So Mannion will lead the Rams in the regular-season finale at the Coliseum.

“This is something I’ve been waiting for my whole career,” Mannion said. “To finally get this first start is really exciting, something everybody dreams of.”

The Rams clinched the NFC West and improved to 11-4 with a victory last week at Tennessee.

Going into Sunday’s games, the Philadelphia Eagles are seeded No. 1, the Minnesota Vikings No. 2 in the NFC playoffs. A victory would earn the the Rams the No. 3 seed. They could fall to No. 4 with a loss.

Mannion, a third-round draft pick from Oregon State in 2015, has waited patiently while quarterbacks such as Nick Foles, Case Keenum and Goff started for the Rams during the past three seasons.

Mannion has played extensively in preseason games but sparingly in the regular season. He has completed 11 of 16 passes for 66 yards in six games. This season he has completed two of three passes for 16 yards.

Sunday’s opportunity is more than a chance to help the Rams finish with their first 12-win season since 2003. Mannion also considers it an audition, not only for the Rams but also for every NFL team that might consider trading for him, or signing him as a free agent when his contract expires after the 2018 season.

“Any opportunity you get to put something on film is great because film is your resume,” he said, adding, “That doesn’t mean you go into it with an individual perspective. The best way to put out good film is to go out and win and play well and help your team.”

Goff has passed for 3,804 yards and 28 touchdowns with seven interceptions, leading an offense that is averaging an NFL-best 31 points a game.

Mannion will face the 49ers with a unit that will be without several key players. Running back Todd Gurley, who leads the NFL in rushing yards and touchdowns, left tackle Andrew Whitworth and center John Sullivan will not play. Rookie receiver Cooper Kupp is sidelined because of a knee injury and receivers Robert Woods and Sammy Watkins will play little if at all.

Darrell Williams, a third-year pro, will start for the first time in place of Whitworth and protect Mannion’s blind side.

“Being able to work with a guy like Whit, you prepare every week like you’re the starter and pick up on some of his tendencies and watch film,” Williams said. “I’ve been just talking to him through the week and asking him what he saw the first time we played [the 49ers].

“I’m feeling confident going into the game.”

Offensive coordinator Matt LaFleur has monitored Mannion’s work habits and preparation since organized team activities last spring. He anticipates that Mannion will perform well.

“We don’t expect any drop-off,” LaFleur said. “The expectations, the standards are going to stay the same with him as they are for Jared.”

Mannion and Goff spend hours together in meetings, reviewing opponents and on the field. Goff credited Mannion for helping him during weekly preparation and in games, and he will return the favor.

“I’m excited to see him go out there, I know we all are,” Goff said. “See him get a chance to really show what he can do.”

With Woods, Watkins and Kupp limited or out, receivers Josh Reynolds, Mike Thomas, Pharoh Cooper and Tavon Austin are expected to be Mannion’s main targets.

Goff’s passes are “a little more arc based,” Mannion’s “a lot more velocity based,” Reynolds said.

“Sean’s going to get it there every single time in the matter of a snap,” Reynolds said. “He’s going to put it on you and you just got to be ready when Sean’s delivering the ball.”

After Sunday’s game, Mannion will revert to a backup role and the one-play-away mentality that No. 2 quarterbacks must adopt and endure.

Mannion got a close-up look at the reality of that situation in Week 14 when Philadelphia quarterback Carson Wentz sustained a season-ending knee injury against the Rams at the Coliseum. Foles stepped in, led the Eagles to victory and is now the Eagles’ starter.

“When it happens in a game you’re playing, I think you realize it is real,” Mannion said.

On Sunday, though, Mannion won’t be one play away. He will be under center from the first snap.

“I just want to provide clean, efficient quarterback play; be an efficient passer with great communication,” he said. “And go out and win the game.”

gary.klein@latimes.com

Follow Gary Klein on Twitter @latimesklein