Jansen sprained his left knee in last week's 24-10 loss to the Baltimore Ravens. He took part in some drills on Thursday but was too sore to participate on Friday.
Jansen was one of three offensive linemen hurt against the Ravens. Left tackle Chris Samuels is out for the season with a torn right triceps, and backup Justin Geisinger is also done for the year with a knee injury.
Second-year player Stephon Heyer will start for Samuels on Sunday, while veteran Jason Fabini is expected to start for Jansen.
Horton has not practiced all week because of a shoulder injury. Mike Green is expected to start at strong safety.
Coach Jim Zorn said he expects middle linebacker London Fletcher (foot) to play, while defensive tackle Cornelius Griffin (shoulder, abdomen) will be a game-time decision. Both did not practice Friday and were listed as questionable.
Copyright 2008 by The Associated Press