The Steelers signed Haynes and placed Parker on injured reserve. Parker had surgery Friday to repair his broken right fibula and should be okay to play next season.
"I wouldn't count Willie Parker out, any form or fashion," coach Mike Tomlin said.
The Steelers' stable of backs now includes Haynes, Davenport, Carey Davis and Gary Russell. Davenport signed with the Steelers last season after Green Bay released him. Davis made the roster after spending time with various teams and playing briefly with the Colts. Russell is an undrafted rookie from Minnesota who did not play college ball in 2005.
Haynes has played halfback, fullback and third-down back, the latter his primary job during the past three seasons.
Davenport will start at halfback, Russell will back him, Davis is the fullback (Dan Kreider is on injured reserve) and Davis and Haynes could both play on third downs.
"Carey has already shown that he is a capable third-down guy," Tomlin said. "Verron made his reputation in the league as that kind of guy.
"It is going to have to be a running back by committee, if you will. I think that's what you deal with when you lose a player with the quality of Willie Parker. Again, it is football. I know those guys are professionals. I know they are serious competitors. I know they look forward to our team not missing a beat and their roles expand."
--OT Marvel Smith probably will miss his fourth game in the past six with a disk problem in his back. Coach Mike Tomlin said it's also possible he could miss the playoffs.
--SS Troy Polamalu could miss Sunday's game in Baltimore because his knee is swollen.
--QB Ben Roethlisberger has a sprained right ankle but it should not prevent him from playing, although coach Mike Tomlin raised the issue that Charlie Batch might see more action in a game that has little bearing on the team's playoff position.
--LB Clint Kriewaldt, the special teams captain, has shoulder "issues," according to coach Mike Tomlin, and could miss the game in Baltimore.
--LB Andre Frazier, who plays mostly on special teams, has a groin injury and Tomlin listed him as questionable.