The 49ers didn’t necessarily think they would be counting on Reuben Foster to start right away.

“It’s going to be hard to keep him off the field. But we’re gong to make him earn it too,” general manager John Lynch said Friday on NFL Network.

San Francisco traded back into the first round of April’s NFL draft to take the Butkus Award winner 31st overall weeks after giving former Super Bowl MVP Malcolm Smith a five-year contract worth up $26.5 million. Smith was the most expensive addition to the defense after it finished dead last in opponents’ yardage and scoring in 2016.

Smith, 28, was expected to help the team’s young defensive front transition to coordinator Robert Saleh’s 4-3, which Smith ran in Seattle when Saleh was an assistant helping to win the championship after 2013.

But Smith during practice Saturday felt pain when he went to push running back Matt Breida out of bounds on a sweeping run to the right. Smith’s arm knocked against Breida’s helmet, and it looked as though Smith knew right away something was wrong. He stayed in for one more play before tapping his helmet and taking himself off the field.

Smith was replaced by Ray-Ray Armstrong, who spent the day playing Smith’s position with the second team while Foster worked at Bowman’s spot.

The 49ers found out later Smith suffered a torn pectoral and is likely done for the season. And even though Armstrong was the first to replace him, the starting job next to Bowman will be Foster’s to lose.

Foster began training camp playing WILL linebacker with the third team, while Armstrong and Brock Coyle played on the second string behind Smith and Bowman.

Foster made his debut at MIKE when Bowman was given a rest on Thursday, allowing Foster to make the calls in his debut with the starters when Coyle left practice early with a rib injury.

“I thought he did a good job,” 49ers coach Kyle Shanahan said afterwards. “We just threw him in there and he started moving a lot. He’s got to make all the calls. I think like anybody in his first time at it, I think he had a few busts. I think there’s good stuff for him to get in and watch the tape and learn from.”

The 49ers want their inside linebackers to be interchangeable, which should help Foster. However, Foster’s skillset is more similar to Bowman’s than Smith’s. Smith is a speedy coverage linebacker better equipped to run with running backs and tight ends. Foster does his best work attacking the offense either as a blitzer or filling lanes against the run.

“Reuben can definitely come downhill and smack you,” Bowman said Saturday. “I think at linebacker you have to smart, patient, know when you can shoot your gun and go make those plays.”

Foster didn’t register an interception with Alabama in his 39 games, which was just about his only knock as a prospect between the lines. However, through eight camp practices, Foster made three interceptions in team drills, with two coming against undrafted rookie quarterback Nick Mullens.

He’s flashed in coverage, at times, which is impressive considering he was held from 11-on-11s throughout the offseason program while recovering from shoulder surgery. Foster’s torn rotator cuff was a leading reason he fell to the bottom of Round 1 after being touted as a top-10 selection and linked to San Francisco picking second.

Reuben Foster gets a PBU on a Nick Mullens pass to Joe Williams. #49ers pic.twitter.com/4MZ6B4SnMQ — Chris Biderman (@ChrisBiderman) August 5, 2017

Now Foster becomes a focal point of the defense. And with Smith hurt, the lurking idea of Bowman being available in a trade takes a backseat. Foster’s job in 2017 will be to replace Smith.

Which, ironically enough, eases a bit of looming tension. It was looking like the 49ers would have had a difficult decision to start the regular season. Keep Foster, the prized rookie, on the bench behind Smith and Bowman? Or bench Smith, the $26-million player? Or trade Bowman to allow Foster to take his spot?

The answer emerged earlier than expected on Saturday. Foster will start next to Bowman – forming one of the league’s most talented linebacker duos.