How do 49ers replace Aldon Smith?

SANTA CLARA, Calif. -- Now that linebacker Aldon Smith is out for likely a month as he seeks treatment for a personal matter, where does that leave the San Francisco 49ers' defense?

Smith is on the reserve/non-football-illness list. He can return when cleared by physicians. The 49ers (1-2) play Jacksonville in London in Week 8, on Oct. 27; their bye week follows. Smith can come off the reserve list at any time -- something that is only allowed during the season if the player has substance-abuse issues. The NFL Players Association is also keeping abreast of the matter, as is customary in these situations.

Smith is a supreme pass-rusher. He had 37 sacks in 35 games since the beginning of his rookie season in 2011 (tops in the NFL during that span), including 3.5 this season. The 49ers will have to pick up the slack elsewhere.

San Francisco coach Jim Harbaugh said linebackers Ahmad Brooks, Dan Skuta and rookie Corey Lemonier will be called on; Skuta is listed as the starter in Smith’s absence.

Defensive lineman Demarcus Dobbs can help in nickel-defense situations. The team could also sign a pass-rusher. Regardless, it will be tough for any of those players to have Smith's impact. He was a sack threat on nearly every passing down.

The team will need a complete defensive effort without Smith. ESPN analyst Matt Williamson believes Smith’s absence could expose the 49ers secondary.

“They definitely have problems and I do worry about their pass rush quite a bit,” Williamson said. “I think it has masked a very average secondary over the past few seasons.”

The San Francisco defense allowed 84 points in three games with Smith and it has been vulnerable, especially in the running game. Now without Smith, San Francisco has to find a way to hold defensively. But there are no easy answers.