The good news is cornerback Keenan Lewis returned to the practice field Thursday after missing Wednesday's session with a hip injury.
Allen (groin) missed his second consecutive practice, bringing into light the possibility Curtis Brown may make his first career start Sunday in a road game against the Dallas Cowboys - one of the better passing teams in the league.
A product of Gilmer, Texas (two hours east of Dallas), Brown played for the University of Texas before being selected in the third round of the 2011 NFL Draft.
Brown was targeted often by Phillip Rivers and the San Diego Chargers in Pittsburgh's Week 14 loss, particularly on third downs, where they spread the field with receivers, bringing Brown - the Steelers' nickel defender - onto the field.
Whether Brown is in the starting lineup (if Allen is unable to play) or remains the nickel back, it's logical to assume Dallas will take on a similar strategy as did the Chargers. Reports indicate wide receiver Dez Bryant will play despite a fractured left index finger that requires surgery. Between Bryant and tight end Jason Witten (92 receptions), they have 46.6 percent of the Cowboys' receptions this season, and 43.8 percent of the team's targets (Bryant 109, Witten 126).
CB Ike Taylor (ankle) has already been ruled out, as has left guard Willie Colon (knee) and right tackle Mike Adams (ankle). Safety Troy Polamalu has been held out of practice as a precautionary measure, and likely won't practice Friday.
Outside linebacker LaMarr Woodley looks set to play in his first game since suffering an ankle injury in Week 12. He's practiced fully Wednesday and Thursday this week.
OLB James Harrison was held out Wednesday and Thursday with an illness but is expected to play.
WR Emmanuel Sanders missed Wednesday due to a shoulder injury he's been battling since Week 13, but returned to full practice Thursday.