SAN FRANCISCO -- Outside linebacker Aldon Smith was placed on the 49ers' reserve/non-football-injury list Monday as he entered treatment after his recent drunken-driving arrest.

Smith's timetable for returning is undetermined, but it might be a month, ESPN first reported. He can be activated by the 49ers at their discretion. According to a league spokesman, there is no minimum length of time that he has to be out.

Also unknown was Smith's destination for treatment. He reportedly was flying to a rehabilitation center Monday morning, according to ProFootballTalk.com, but coach Jim Harbaugh declined to confirm that.

In a team-issued statement, Smith said: "I am taking a leave of absence to address my health. 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."

Harbaugh also declined Monday to "rehash" the 49ers' decision to play Smith in Sunday's 27-7 loss to the Indianapolis Colts.

"We're supportive of Aldon, as we are of all players, all of our coaches, our personnel. Always supportive of in-house," Harbaugh said. "We've hashed through it and now taken steps, and Aldon's taken steps. We support him in that."

General manager Trent Baalke said in a statement: "Aldon Smith is taking a leave of absence to address a personal matter. The 49ers organization both respects and supports Aldon's decision and commends him for taking responsibility for his actions."

Smith still likely faces a suspension from the NFL, which has not commented on Friday's arrest. Such discipline might be delayed until Smith's criminal case is resolved, however. His arraignment is Nov. 4.

The 49ers have not yet decided whether to dock Smith's pay, according to a source who noted such a move might depend on potential fines the NFL imposes on him. Smith has a base salary of $1.68 million this season.

The 49ers likely will use a committee of defenders to fill in at right outside linebacker and right defensive end for Smith, who played all 72 defensive snaps Sunday. Three players stand out as likely replacements:

  • Dan Skuta: He was listed as a starter on Thursday's depth chart. After four years with the Bengals, Skuta looked solid in the exhibition season and had three sacks. But he hasn't taken a defensive snap through three regular-season games (46 special-teams snaps).

  • Demarcus Dobbs: A third-year veteran, Dobbs is the top backup on the defensive front. Suspended for the opener because of his own alcohol- and marijuana-involved car accident, he's rotated in the past two games, typically spelling tackles Justin Smith and Ray McDonald. He has no career sacks.

  • Corey Lemonier: The rookie, third-round pick likely would be used strictly as a pass rusher. He hasn't had any snaps on defense, but 24 on special teams. He had two sacks in the exhibition season.

    "There's adversity. There's an opportunity to stare it in the face and whip it," Harbaugh said.

    Smith had no sacks Sunday against the Colts.

  • Harbaugh said linebacker Patrick Willis (groin) and cornerback Nnamdi Asomugha (right knee) were having MRI exams Monday.

    New to the injury report: running back Frank Gore (knee), right tackle Anthony Davis (shoulder), center Jonathan Goodwin (elbow), wideout Kyle Williams (rib) and cornerback Carlos Rogers (knee). Quarterback Colin Kaepernick no longer is listed with a foot injury, nor is running back LaMichael James (knee).

  • Harbaugh said Gore had no physical ailment that prevented him from having more than three carries Sunday after a strong first half. Asked why Gore didn't get more carries, Harbaugh replied: "Those were the plays that were called."

  • Wide receiver Jon Baldwin might debut Thursday in St. Louis after what Harbaugh called "very good" practices last week. Baldwin was acquired in an August trade with the Kansas City Chiefs for A.J. Jenkins. Rather than chastise his receiving corps for its recent lack of production, Harbaugh said the entire offense's "precision needs to be better."

  • Harbaugh said he remains "very confident" in Kaepernick, whose confidence he said remains unshaken after two poor passing performances.

  • The 49ers are in the midst of their first two-game losing streak since Harbaugh became coach in 2011. "We've got some adversity, there's no question about it," Harbaugh said.

    Staff writer Tim Kawakami contributed to this report.