Montgomery may need reconstructive surgery on the knee, the same one he injured in practice near the end of last season when he missed the final three games. If that's the case, he will be sidelined indefinitely.
"It's the same knee he injured in high school," McCarthy said. "When you get into that there are some things you want to look at, obviously."
Montgomery has been playing as one of the top backups on the defensive line at end.
Without Montgomery, Jason Hunter and defensive tackle Corey Williams are expected to fill in at end in place of Montgomery. Williams played six snaps at defensive end against the Seahawks, McCarthy said.
Wide receiver/kick return specialist Shaun Bodiford and fullback Ryan Powdrell also will be out about three to five weeks due to MCL injuries they sustained against the Seahawks last Saturday. McCarthy said that offensive tackle Orrin Thompson aggravated his MCL injury against the Seattle, but won't be out as long as Bodiford and Powdrell.
Aside from the aforementioned players, the following did not practice Monday due to lingering injuries: running backs Vernand Morency (knee), P.J. Pope (knee), DeShawn Wynn (quadriceps); offensive linemen Tony Palmer (hamstring) and Tony Moll (stinger), and defensive lineman DeVon Hicks (groin).
Defensive end Kabeer Gbaja-Biamila left practice early on Tuesday to attend a "family situation" in Milwaukee, according to McCarthy. Gbaja-Biamila also is nursing a bruised knee and it is uncertain if he will practice on Tuesday.
"He'll be day to day," McCarthy said.