Lewis missed last Sunday's game against the Bengals, but he participated in every practice over the last three days. Head coach John Harbaugh essentially said earlier today that Lewis will be a gametime decision.
Also listed as questionable for the Ravens are linebacker Dannell Ellerbe (hamstring), cornerback Chris Carr (back) and running back Anthony Allen (hamstring).
Defensive tackle Arthur Jones (concussion) has been ruled out for the second straight game, while safety Haruki Nakamura (thigh) is probable for Thursday night.
For the 49ers, five players are listed as probable: running back Frank Gore (knee), tackle Anthony David (ankle), CB Chris Culliver (shoulder), wide receiver Michael Crabtree (foot) and wide receiver Braylon Edwards (knee).
Fullback Bruce Miller (concussion) is doubtful for San Francisco.