SANTA CLARA (AP) — Running back Marcus Lattimore never played an NFL snap because of two serious knee injuries while he was in college, and he has decided his knees might not hold up through the rigors of a 16-game season.

Lattimore informed the San Francisco 49ers he will retire, two years after blowing out his right knee at South Carolina and working tirelessly to try to come back.

“After prayer and careful consideration, I have decided it’s time to end my professional football career,” Lattimore said in a statement released by the team. “I have given my heart and soul to the game that I love, and it’s time for me to move on to the next chapter of my life and help others.

“I have given every ounce of my energy toward making a full recovery from my knee injury, and I have made a lot of progress. Unfortunately, getting my knee fully back to the level the NFL demands has proven to be insurmountable.

“I am grateful for the entire 49ers organization. Their decision to draft me was the realization of a lifelong dream to be an NFL player, and I cannot thank them enough for believing in me and for doing everything in their power to assist in my recovery. I will always be a 49er!”

The team made the announcement Wednesday, two days after coach Jim Harbaugh said Lattimore was weighing his football future. Meanwhile, South Carolina coach Steve Spurrier said he told Lattimore there will be a position for him at his former university.

Harbaugh said he and Lattimore spoke last week and during the weekend.

“He gave it his best shot,” Harbaugh said. “There’s a trusting, lasting friendship there.”

The 23-year-old Lattimore, San Francisco’s 2013 fourth-round draft pick, practiced for the first time last Wednesday and Thursday before sitting out Friday. He was eligible to return after Week 6, but Harbaugh said Lattimore had doubts his knees would allow him to play at the highest level long term.

“I talked to him. He made his mind up,” running back Frank Gore said. “It’s tough, man.”

Lattimore spent his rookie season last year on injured reserve after tearing major ligaments in both knees during college playing for the Gamecocks. The serious knee injuries derailed his 2011 and ‘12 seasons before the Niners took a chance on him in the draft anyway.

“I was looking forward to seeing him on the field, seeing what he could do,” quarterback Colin Kaepernick said.

He underwent surgery for a torn anterior cruciate ligament in his left knee in 2011, then injured the right knee in October 2012.

“It’s a tough business,” veteran place kicker Phil Dawson said. “We’re all walking a fine line here. You spend so much time training and dreaming and working and one play can change the course of your entire career. Marcus is a great guy. He’s got a lot going for him. I think he’ll be successful in whatever he decides to do, but it makes those of us who are still playing grateful for the opportunities we have each and every day.”