The San Francisco 49ers running game is relying on undrafted running backs Matt Breida, Raheem Mostert and Jeff Wilson Jr.

Running backs Jerick McKinnon and Tevin Coleman were two of the 49ers' significant signings after the arrivals of general head coach Kyle Shanahan and general manager John Lynch.

McKinnon has not played a down for the team and has spent the past two seasons on injured reserve after an ACL injury in 2018. Coleman has been week-to-week with an ankle injury since the Week 1 victory against the Tampa Bay Buccaneers.

Breida signed with the 49ers in 2017 out of Georgia Southern and was a capable backup behind former 49er running back Carlos Hyde. He was thrust into starting last year but was hobbled by an ankle injury.

During a phone interview on the "Murph and Mac" show on KNBR, Breida said he wanted to concentrate on his ankle during the offseason and make sure he stays healthy for the year.

Brieda told the hosts that he, Mostert, and Wilson are like brothers and feels they have each others' backs.

"As far as the undrafted thing, I feel all of us got a chip on our shoulder coming in wanting to prove we belong in the NFL," Breida said. "I feel like it makes us stay hungry."

In the press conference on September 16, Shanahan told reporters that Breida is the best at what he does because he is very efficient.

"Breida's as tough as any running back I've been around, but it's also very important to keep him fresh," Shanahan said. "He was able to play with us a lot last year, but Matt, just like anybody, the more he gets worn down, the harder it is to play up to his full ability. That's why we do try to spell him, try not to keep him in there every single down, because we believe it'll help him, which helps the offense in the long run."

Shanahan said he was hard on Mostert after he lost a fumble during a Week 5 home loss against the Arizona Cardinals in 2018, but the running back wanted to prove he was more than a special teams player.

"I know Raheem, going into this offseason, I think he showed us and everyone else that he can make those plays, but I think he also showed himself," Shanahan said. "I know going into this offseason, he had a lot more confidence because he always thought he could do it, but he actually went in and did it for a few games. His opportunities didn't come right away, but when Tevin got hurt, since he went in versus Tampa and what he did yesterday, it was exactly, to me, how he left off versus Green Bay."

Breida had a menial game against the Buccaneers with 37 rushing yards but bounced back with 121 yards on 12 attempts against the Bengals. Mostert had 83 yards on 13 rushing attempts and 68 yards receiving on three receptions with a 39-yard touchdown reception in the same game.

The 49ers had a total of 259 yards rushing against the Bengals with the three undrafted running backs.

Breida averaging 5.9 yards and Mostert averaging 5.6 per rushing attempt ranks both teammates in the top 10 in the NFL when it comes to rushing yards per attempt.

"Our mindset is how it always been, and that is control what we can control and just go out there and put our best foot forward," Mostert said on 49ers Live this week. "That is all our coaches ask of us. Even Bobby [Turner, 49ers running back coach], that is what he asks of us. He always tells us when your number is called, grab the ball."

NFL history is known for having running backs such as Priest Holmes, Arian Foster, and the 49ers' very own Joe Perry, who went undrafted and became all-time great rushers. With Coleman and McKinnon out, the three undrafted running backs have the 49ers ranked fourth in the league with 178.5 rushing yards per game.

The 49ers' next game will be their home opener against the Pittsburgh Steelers. Through two games, the Steelers defense has allowed an average of 125 rushing yards per game.