The extent of Ngata's injury is not yet known.
Also missing practice for the Ravens today were wide receiver Lee Evans (ankle), wide receiver/kick returner David Reed (knee) and linebacker Dannell Ellerbe (hamstring).
Grubbs is listed as being a limited participant in today's practice. Joining him in that category were center Matt Birk (neck), cornerback Danny Gorrer (thigh) and running back Anthony Allen (thigh).
For the Steelers, both starting guard Doug Legursky (toe) and reserve wide receiver Emmanuel Sanders (knee) did not participate in today's session.
Four Pittsburgh starters are listed as having a limited practice: linebacker James Farrior (calf), linebacker James Harrison (eye), wide receiver Hines Ward (ankle) and linebacker LaMarr Woodley (hamstring).
Reserve offensive tackle Jonathan Scott (ankle) was also limited.