"Todd is probably a little bit better in run support," Bears coach Lovie Smith said. "We feel like it's going to be a physical football game this week."
Johnson, who spent his rookie season in 2003 on injured reserve with a fractured jaw, started 10 games at free safety last season. Green has started 42 games for the Bears, including all 16 last season and in 2002. Johnson started Game 4 this season when the Bears opened with three safeties.
"I was just disappointed for Chris that he had to go down like that," Johnson said. "I'm just going to try to fill in and pick up where he left off. I feel comfortable out there, especially with (strong safety) Mike Brown kind of taking control and putting people in the right place."
Harris walked without a limp Wednesday, but he was listed as doubtful for Sunday because of the sprained MCL in his right knee he suffered against Green Bay.
"It's nothing major, nothing that will require surgery," Harris said. "I probably won't play this week, but I want to get back as fast as possible. Maybe next week, maybe the week after."
Harris said he suffered the same injury in college and missed just one game.
The sixth-round pick was moved into the starting lineup ahead of Green after the first game of the season and is seventh on the team with 61 tackles.
"Those are the two that's gotten the most work with it," Smith said. "We have some other players that can catch the football.
"We're concentrating on improving these two guys now."
Among the group that stayed after practice were Eddie Berlin and Bernard Berrian. Both have returned punts in the NFL and could be called upon if either Wade or Davis continue to have problems with ball security.
Third Down and Out
The Bears' 0-for-10 performance in third-down situations Sunday against the Packers dropped them to No. 31 in the NFL in that category with a season success rate of just 28.5 percent.
Not only did the Bears fail to convert any of their third-down opportunities against Green Bay, they netted just 3 yards on those 10 snaps, for an average of 1 foot per play. On eight passing plays, the Bears had a net of minus-1 yard. Quarterback Kyle Orton completed two of seven third-down passes for 8 yards and was sacked once for a 9-yard loss.
RB Cedric Benson (sprained knee) continues to run on the side at practice, but only in a straight line. He's listed as doubtful but will not play on Sunday.