49ers’ Brandon Thomas making push to be starter

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First, his knee hurt. Then, his heart ached.

Now, 49ers guard Brandon Thomas is in a better place: He’s dealing with less pain as he prepares for his delayed NFL debut.

A year ago, Thomas was in the midst of an excruciating seven-month stretch in which he sustained another torn ACL and lost his mother, Yvonne, to a blood cancer, multiple myeloma, at 60.

Last week, Thomas reflected on his difficult 2014. He tore the ACL in his right knee in a workout with the Saints about a month before the draft and the injury made the potential second-round pick the final pick of the third round (the 49ers took him at No. 100). Thomas spent his first year rehabilitating at the 49ers’ facility, but left the team in November to be with his mother in the days preceding her death.

Thomas, 24, is grateful his mom, who made it clear education ranked far above athletics, was present when he graduated from Clemson and entered the NFL. His guess: She was a bit more excited about that diploma than about draft day.

“Those were huge events and she was able to be there for them,” Thomas said. “She won’t be there when I play in an NFL game, but I know she’ll be with me.”

Thomas will play in his first game Saturday when the 49ers visit Houston in their preseason opener. It will mark the first time he has played in a game since he participated in the Senior Bowl more than 18 months ago.

San Francisco 49ers' offensive lineman Brandon Thomas, 60 during training camp at their practice facility in Santa Clara, Calif., on Fri. August 7, 2015. San Francisco 49ers' offensive lineman Brandon Thomas, 60 during training camp at their practice facility in Santa Clara, Calif., on Fri. August 7, 2015. Photo: Michael Macor, The Chronicle Buy photo Photo: Michael Macor, The Chronicle Image 1 of / 1 Caption Close 49ers’ Brandon Thomas making push to be starter 1 / 1 Back to Gallery

Given his pedigree, the two-time All-ACC pick was viewed as a candidate to fill the vacancy created in March when Pro Bowl guard Mike Iupati signed with Arizona. However, Thomas has spent the majority of the first eight practices of training camp with the second unit.

That’s partly a reflection on first-string guard Marcus Martin, another 2014 third-round pick who made eight starts at center as a rookie. And it’s also a nod to the rust Thomas is knocking off. In addition to returning from a serious injury, he’s playing guard for the first time since 2011.

Thomas, a left tackle for his final two seasons Clemson, opened eyes in 2013 when he went mostly one-on-one with South Carolina defensive end Jadeveon Clowney, and the No. 1 pick in 2014 finished with two solo tackles and a sack.

On Sunday, left tackle Joe Staley said Thomas (6-foot-3, 317 pounds) was starting to show why he generated such pre-draft buzz.

“His first week kind of coming back — he had a year off from football, basically — so that transition is going to take a little bit,” Staley said. “But you’re starting to see more of the natural talent that he has and the reason that we drafted him high last year. ... He’s so powerful, and he’s so big and moves really well. And you’re seeing that come on in the second week.”

Thomas has been here before. He tore the ACL in his left knee as a senior at Dorman High in Roebuck, S.C., and the injury delayed his college development. He redshirted in 2009, played only 136 snaps in 2010 and didn’t become a starter until the fourth game of his sophomore season.

“It’s kind of the same situation and I know it’s going to be a process,” Thomas said. “I’m sure everyone knows it’s going to be a process. I’m working hard to push through every day, trying to come out, be competitive and learn what I need to learn.”

Thomas is eager to assist the 49ers, who surprised him when they halted his draft plunge. After his ACL tear, Thomas said teams began canceling the pre-draft visits they’d scheduled and he feared he’d also be ignored until deep in the draft.

Now, more than a year after he was selected, he’s finally in a better place: A spot where he can start to show the 49ers that they’ll be rewarded for their risk.

“I was surprised and happy when they” drafted me, Thomas said. “But then I thought, ‘I’ve got to go to work now and prove that I can help. I’m not just going to be another pick.’”

Eric Branch is a San Francisco Chronicle staff writer. E-mail: ebranch@sfchronicle.com Twitter: @Eric_Branch

Changing of the line

It’s not clear if Brandon Thomas or the front-runner, Marcus Martin, will start at right guard, but one thing is evident: The offensive line will look different this season:

2014 starters

LT Joe Staley

LG Mike Iupati

C Daniel Kilgore*

RG Alex Boone

RT Anthony Davis

Camp first-string

LT Joe Staley

LG Alex Boone

C Joe Looney

RG Marcus Martin

RT Erik Pears

* Still recovering from a broken ankle he sustained in October and is on the physically unable to perform list.