As reporters walked from the media room to a nearby interview room Monday to speak with Reuben Foster, they were greeted warmly by the 49ers rookie inside linebacker.

In a rarely seen move, Foster stood outside the door and offered each scribe a smile and a handshake before they entered. And it wasn’t surprising that the ebullient first-round pick was in a particularly good mood: Before his receiving line, he’d seemingly received word that the starting spot alongside All-Pro NaVorro Bowman was his to lose.

In the 49ers’ first practice since inside linebacker Malcom Smith sustained a season-ending injury Saturday, Foster, the No. 31 overall pick, took all the first-team snaps. On Saturday, after Smith was injured, Ray-Ray Armstrong played with the first team instead of Foster.

“It’s meaningful, but in this league you have a lot of great players behind you,” Foster said. “So it’s not like I’m just going to step ahead like I’m (a starter). I’m just looking it at like it’s a learning process.”

The player directly behind Foster is Armstrong, a former undrafted free agent with five starts in his three-team, four-year career. Meanwhile, Foster was rated by the 49ers as the third-best player in draft after he was a unanimous first-team All-American and won the Butkus Award as the nation’s best linebacker last year.

In other words, the 49ers are expecting Foster to win the job, although defensive coordinator Robert Saleh said Armstrong would be a part of their evaluation process. Armstrong said he expects to be given a fair shot.

“No doubt,” Armstrong said. “Obviously, he’s a first-round pick. But I go out and compete every day just like everybody else and then let the coaches make the decision when it’s time.”

The coaches will have to make a decision because Smith, who signed a five-year, $26.5 million deal in March, suffered a torn pectoral muscle in practice Saturday. Despite the team’s excitement over Foster, they had been thrilled with early returns on Smith, 28, a former Super Bowl MVP who had a mastery of the defense and was in the midst of an excellent training camp.

Now, instead of being eased into his rookie year, Foster could be learning on the job. And his learning curve is even steeper after he missed the offseason program while he rehabilitated from shoulder surgery.

Foster has three interceptions and a fumble recovery he snagged in mid-air in training camp, but he’s also had some more subtle lowlights, partly due to his aggressiveness.

“He’s had flash plays and he’s had busts that the common person won’t notice,” Saleh said. “At the same time, he’s been asked to learn a lot in every short amount of time.”

Foster has given Saleh reason to believe he’ll be ready for the season opener against Carolina on Sept. 10. Saleh said he doesn’t repeat mistakes and has displayed a better-than-advertised football IQ.

“He needs reps,” Saleh said. “He’s still a rookie. The game is faster, the linemen are faster, they’re stronger. The scheme is different. So he just needs reps, over and over again. He needs to get attacked in all the different fundamentals that we’re asking him to do. He needs to get attacked in those situations just so he understands what his weaknesses are within the scheme.”

Foster isn’t even officially an NFL starter yet, but he’s shown star qualities in interview sessions that routinely inspire laughs.

On Saturday, as he recalled being chewed out by Alabama coach Nick Saban as a freshman, he was asked for details of Saban’s diatribe. He responded by smiling, raising his hands and looking into the television cameras – making it clear he wouldn’t subject reporters and his wider audience to the expletive-filled rant.

He also marveled at Bowman’s calm in situations that have Foster panicking (“It freaks me sometimes”), and expressed admiration for what he termed Smith’s “famous peanut punch.”

Peanut punch? It’s a move Smith has used to force fumbles throughout his career. Foster said it’s one of the many things he needs to learn as he readies for his first NFL season.

“I’ve tried it plenty of times,” Foster said. “Every time I go for the ball I try it, but no success, though. He’s the master of it. That’s his signature move.”

Eric Branch is a San Francisco Chronicle staff writer. Email: Twitter: @Eric_Branch

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