Peppers suffered a sprained MCL in the first half of the Bears' 24-13 loss to the Lions on Monday night, but did return to finish the game.
"We downgraded Julius Peppers to doubtful ... he wasn't able to do anything today," Bears coach Lovie Smith said following practice. "We are still hopeful, (but) when a guy doesn't practice at all during the week we're not too optimistic, but we'll see."
Being officially listed as doubtful means there is only a 25 percent chance a player will be active, but Smith was careful not to completely rule out Peppers.
As the Bears witnessed firsthand last year, teams do sometimes play tricks with the injury report. For example, then-Minnesota quarterback Brett Favre was ruled out leading up to the game, but was eventually elevated and started against the Bears.
Peppers' absence would severely hurt the Bears' chances on defense, especially because the unit has struggled to generate consistent pressure on opposing quarterbacks. Peppers and Israel Idonije are tied for second on the club with two sacks each.
"Great player you have to deal with, especially in passing downs you have to think about putting two guys on him and all those things," Smith said.
However, a league source said earlier in the week Bennett will not play against Minnesota, but could be cleared to resume contact drills next week.
Jeff Dickerson covers the Bears for ESPNChicago.com and ESPN 1000.