SANTA CLARA -- Glenn Dorsey called it the "magic question:" How does he know when the Seattle Seahawks will try cutting his legs from underneath him Sunday?

"You have to move your feet and be aware at all times," Dorsey said Thursday.

Dorsey took over as the 49ers' top nose tackle when the Seahawks broke Ian Williams' ankle on a cut block Sept. 15, when the visiting 49ers fell 29-3.

"It's a part of the game and a part of their scheme," Dorsey added. "You have to be aware and cognizant that someone is coming in to cut block you."

Seahawks coach Pete Carroll noted Wednesday it was a legal move "and nothing wrong" with it other than an "unfortunate" injury.

"There may come a time the competition committee frowns upon it," Carroll said of the cut block. "We have a lot of rules we're adjusting to. I wouldn't be surprised if that comes about. That's a situation that's still tough on defensive linemen."

Defensive tackle Justin Smith said the 49ers must be conscious and prepared for Seattle's cut-block tactics in their zone-blocking scheme. Smith didn't condemn the legality of such blocks, noting: "Inside the tackle box, it's fair game."

Williams is the only 49ers starter who's gone on season-ending injured reserve. His exit opened the door for Dorsey, the 49ers' first free-agent signing last March.

Dorsey has come on strong and had his best game of the season in Sunday's 23-13 win over the St. Louis Rams, according to defensive coordinator Vic Fangio. Dorsey produced a season-high 13 tackles.

Dorsey, a former first-round pick of the Kansas City Chiefs, agreed that he's coming off his best game since switching to nose tackle with the 49ers. "But you watch and see stuff you should do better," Dorsey said. "There's a lot I can still improve."

He said Sunday's task will be to keep quarterback Russell Wilson "caged in" and also shut down hard-charging rusher Marshawn Lynch. "It's really going to be won up front," Dorsey said.