Pittsburgh Steelers quarterback Ben Roethlisberger, who practiced this week despite suffering the fourth concussion of his career last week, will miss Sunday night's game against the Baltimore Ravens.
Roethlisberger expressed reservations Friday about playing, and he and the Steelers' medical staff decided Saturday it'd be best if he sat out.
To make room for Palko, the Steelers released receiver Shaun McDonald.
Dixon, the third-string option before Charlie Batch broke his left wrist when he came in during last Sunday's loss to Kansas City, has appeared in one game in his two NFL seasons, completing one pass.
Roethlisberger, who was having his best season statistically despite the Steelers' erratic play, went through "a normal week" of preparation for the pivotal game at Baltimore and had said he had every intention of playing.
The Steelers decided Saturday that Roethlisberger will play only in an emergency. He will be the No. 3 quarterback, with former Pitt starter Palko -- who was signed only Thursday and doesn't know the offense -- as Dixon's backup.
"It's part of the nature of the beast of playing this game. It's a violent, physical contact sport and there's a chance you're going to get hit," Roethlisberger said Thursday. "You guys don't talk about the bruises we have all over our body. If I showed you a bruise on my shoulder and a bruise on my shin, it wouldn't get talked about as much. It's a violent sport we play."
The last time the Steelers and Ravens met, a berth in the Super Bowl was at stake. This time, the matchup could play a big role in which team reaches the postseason.
Pittsburgh (6-4) beat Baltimore (5-5) three times in 2008 en route to its record sixth Super Bowl title, and its 23-14 win in the AFC title game may have been the most bruising encounter of the bunch.
The defensive battle saw each team punt seven times, with the Steelers putting the game out of reach with Troy Polamalu's 40-yard interception return for a touchdown with 4:24 left.
Both teams trail division leader Cincinnati, which is 7-3.
Baltimore failed to score an offensive touchdown for the second straight game last Sunday, falling 17-15 to unbeaten Indianapolis. Pittsburgh's loss last Sunday was much more surprising, blowing a 10-point lead in a 27-24 overtime defeat to a Kansas City team which entered with a 2-7 record.
The Steelers' second consecutive loss and the Ravens' fourth in six games helped jumble the AFC playoff picture, and added fuel to this edition of the rivalry.
"You know it's fireworks every time we play these guys, we're excited about preparing for them," Steelers coach Mike Tomlin said. "You know they're a similar team that we remember competing against. They can say similar things that we're saying, that they're a 5-5 team and not happy with where they are."
Getting untracked against the Steelers' top-ranked defense won't be easy for the Ravens, whose losses have all come against current division leaders.
"These losses have been against teams that are quality football teams," coach John Harbaugh said. "Call them elite football teams in the league, but we aspire to that."
Flacco had one touchdown, five interceptions and a 44.9 completion percentage while getting sacked 10 times in the three losses to Pittsburgh last season.
The Steelers likely will rely mostly on running backs Rashard Mendenhall and Willie Parker and limit the amount of throws Dixon makes. Dixon was an excellent runner in Oregon's spread offense, but the Steelers -- like every NFL team -- don't want their quarterback running consistently because of the risk of injury.
Dixon's only game action came in a mop-up role against Cleveland in the final game of last season, after Roethlisberger also received a concussion. Dixon will be the most inexperienced Steelers quarterback to be thrown into a starting role since rookie Mike Kruczek replaced the injured Terry Bradshaw in 1976. Kruczek went 6-0 as a starter despite not throwing a single touchdown pass, and ended his five-season NFL career in 1980 without throwing a scoring pass.
Roethlisberger's decision to not play comes in the same week the NFL has taken a heightened stance on protecting players from head injuries. NFL commissioner Roger Goodell sent a memo to teams outlining steps the league is taking to reduce head impacts.
Roethlisberger also had two concussions in 2006, one in a motorcycle accident, and the other last season.
Information from ESPN.com's John Clayton, The Associated Press and STATS LLC was used in this report.