SANTA CLARA Aldon Smith on Monday checked into a rehabilitation clinic and the 49ers placed him on the reserve non-football injury list.
That designation opens a roster spot for the team, which will be short-handed as it prepares to face St. Louis on Thursday night. There is no minimum amount of time that Smith must miss, and he will not return until he is cleared by doctors who are independent of the team.
"The long-term well-being of our players is paramount, and we will stand by Aldon as he works toward addressing this issue," general manager Trent Baalke said in a statement. "Due to the private nature of the matter, we will have no further comment."
Said Smith in a statement: "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."
Smith, who turns 24 on Wednesday, was arrested early Friday morning for DUI and marijuana possession after crashing his pickup truck into a San Jose front yard. In 2012, he was arrested for DUI in Miami Beach, Fla., and stabbed at a house party. Smith is currently being sued by a man who was shot at that same party.
Dan Skuta likely will start at right outside linebacker in Smith's absence with rookie Corey Lemonier filling in on passing downs. In the offseason, the 49ers traded one outside linebacker, Parys Haralson, to the Saints for a seventh-round pick and traded another, Cam Johnson, to the Colts for a seventh-round pick.
Baldwin's turn Aside from Anquan Boldin, the 49ers' wide receiving corps has been quiet this season. Kyle Williams, the team's No. 2 receiver, was targeted six times and finished with two catches for 12 yards and one third-down drop Sunday. Marlon Moore (Natomas High School) and Quinton Patton each were targeted once but had no catches.
Asked whether Jon Baldwin, who has been inactive to this point, could play soon, coach Jim Harbaugh said, "That's possible. He's doing a good job. Had a good week of practice last week."
The 49ers traded A.J. Jenkins for Baldwin, a former first-round pick by the Chiefs, in August. He's 6-foot-4, 230 pounds and has excellent speed. The 49ers might be able to use him to stretch defenses, something they had a hard time doing without tight end Vernon Davis (hamstring) in the lineup Sunday.
Injuries Harbaugh said linebacker Patrick Willis (groin) and cornerback Nnamdi Asomugha (knee) had MRIs Monday, and both are unlikely to play against the Rams.
If Asomugha's injury is serious, cornerback Eric Wright likely would be activated at the earliest possible point, which is before the team's Week 7 game at Tennessee.
Davis was listed as a limited participant after Monday's walk-through. The 49ers filed a rare Monday injury report because this week's game is Thursday.