SAN FRANCISCO >> The San Francisco 49ers are prepared to go forward without star linebacker Aldon Smith for as long as it takes for him to get clean, healthy and improve his life.

Smith was placed on the reserve non-football injury list Monday as he enters rehab for substance abuse and begins what team CEO Jed York has termed an indefinite absence.

“I am taking a leave of absence to address my health,” Smith said through the 49ers. “I am sorry that I have affected my team, my family and the organization. I will do everything in my power to handle this situation the best way possible. I appreciate the support of the 49ers and our fans.”

The NFL said there is no minimum number of games Smith must miss on the reserve non-football injury list, and he can return when ready.

On whether he expects Smith to return this season, coach Jim Harbaugh said only, “there’s a process, we’re going to respect that process.”

Smith played Sunday and had five tackles just two days after he was arrested and jailed Friday morning on suspicion of DUI and marijuana possession. Smith apologized after the 27-7 home loss to the Colts, and now his team moves forward without him heading to St. Louis on Thursday.

“We’re very supportive of Aldon as we are of all of our players, all of our coaches, our personnel. Always support it in house,” Harbaugh said. “We’ve hashed through it, now we’re taking steps and Aldon’s taking steps. We support him in that.”

San Francisco didn’t immediately announce a corresponding roster move.

“We have a pretty good idea what we’re going to do and we’ll make that announcement,” Harbaugh said.

Smith had previously been arrested on suspicion of DUI in January 2012 in Miami shortly after the 49ers lost in the NFC championship game. He is expected to remain in the Bay Area for treatment.

He could face a suspension from the NFL either later this season or next season, and it is likely given he has had more than one infraction. The NFL would let the legal process play itself out first.

Harbaugh said Dan Skuta, Ahmad Brooks and Corey Lemonier will help fill the void left by Smith, who had a franchise-best 19 1/2 sacks last season. Demarcus Dobbs, who was suspended for Week 1 for his own arrest on suspicion of DUI and marijuana possession from last season, also could be in the mix for some packages.

“I have confidence in those guys and know they’ll do a good job,” Harbaugh said. “Go with the guys we’ve got.”

Smith, selected seventh overall in the 2011 draft out of Missouri, turns 24 on Wednesday.

Missouri coach Gary Pinkel said Smith “never had any problems with that, zero,” at school. He said he texted Smith earlier Monday and thought the situation could be turned into something “real positive.”

“You look at any player who’s 22-23 years old and has tremendous success, that’s difficult to handle,” Pinkel said. “You become very, very vulnerable in many, many ways and maybe ways you don’t even understand.”

Harbaugh and general manager Trent Baalke ultimately decided to let Smith play before he entered rehab.

“Aldon Smith is taking a leave of absence to address a personal matter,” Baalke said in a statement. “The 49ers organization both respects and supports Aldon’s decision and commends him for taking responsibility for his actions. The long-term well-being of our players is paramount and we will stand by Aldon as he works toward addressing this issue. Due to the private nature of the matter, we will have no further comment.”

His 49ers teammates were trying their best not to let the situation become a distraction. San Francisco (1-2) has a quick turnaround to face division foe St. Louis on the road Thursday night.

“Coach decided to let him play, and he was out there and made some plays,” safety Donte Whitner said. “I don’t think guys were out there thinking about Aldon being arrested.”

Earlier this month, Smith and former teammate Delanie Walker were named in a lawsuit filed in Santa Clara County Superior Court by a Northern California man who said he was shot at a party at Smith’s house on June 29, 2012.

The players charged a $10 admission and $5 per drink, the lawsuit said. Smith and now-Titans tight end Walker, 29 were allegedly intoxicated on Smith’s balcony when they fired gunshots in the air while trying to end the party, the lawsuit said.

Before the 2012 home opener last September, Smith was the passenger in a car during an accident in Santa Clara County in which the driver swerved to avoid hitting a deer. Smith sustained a cut beneath his right eyebrow. He apologized and insisted he had grown up.

Asked about Smith on Monday at Titans headquarters, Walker said he is getting more attention because of his status as one of the best in the NFL at his position.

“I pray for him because he is a good kid,” Walker said. “He’s just making some bad choices now.”

Missouri defensive end Kony Ealy, named SEC defensive lineman of the week Monday, considers Smith a mentor and hoped it was all a misunderstanding.

“It’s hard to see him just get ridiculed like that by outside media,” Ealy said. “You can’t judge a book by it’s cover, you don’t know what actually happened. Anything could have happened.”

Notes: CB Nnamdi Asomugha (leg) and LB Patrick Willis (groin) were getting MRI exams Monday, while Harbaugh said TE Vernon Davis was being evaluated after he missed Sunday’s game with a hamstring injury. … WR Jon Baldwin, acquired from Kansas City in the trade that sent WR A.J. Jenkins to the Chiefs last month, could be active for the first time Thursday as the 49ers look for more from their receiving corps. They had 13 total catches Sunday. “That’s possible,” Harbaugh said. “He’s doing a good job, had a real good week of practice last week.”

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AP Sports Writer Teresa M. Walker in Nashville, Tenn., and Columbia, Mo., stringer Jake Kreinberg contributed to this story.