Pierce injured his hamstring in the Oct. 20 loss at Pittsburgh and couldn't finish the game, which is why Vonta Leach was in the backfield at times. Pierce also didn't practice in the team's final workout before the players received four days off for the bye.
This is the latest setback for the Ravens' struggling running game. Even though Pierce was averaging 2.8 yards per carry, he did break longer runs this season than Ray Rice.
Scott, 29, is a reliable backup who knows the defenses in the AFC North. A sixth-round pick by the Bengals in 2009, Scott averaged 4.9 yards per carry for Cincinnati in 2010 and scored three touchdowns in 2011.
He is known for his athleticism, but he never received the touches to show it in Cincinnati. Scott was expected to split carries in a running back-by-committee with BenJarvus Green-Ellis last season before tearing his anterior cruciate ligament last October.
Scott is the second former Bengals player to be signed by the Ravens in the past five weeks, joining backup safety Jeromy Miles. The Ravens play the AFC North-leading Bengals in two weeks.
The Ravens didn't have to make a roster move to add Scott because they had a spot open since trading offensive tackle Bryant McKinnie to the Miami Dolphins last week.