Pierce originally joined the Bears last year as an undrafted free agent from Oregon State. He appeared in five games as a rookie, rushing for 34 yards and one touchdown on six carries.
After being waived during final cuts this summer, Pierce was signed to the practice squad Oct. 5, two days after David Montgomery sustained a knee injury in a win over the Lions.
With Montgomery on injured reserve and backup Damien Williams placed on the reserve/COVID-19 list Thursday, the Bears promoted Ryan Nall from the practice squad and flexed Pierce up to the active roster. Nall and Pierce were teammates at Oregon State.
Rookie sixth-round pick Khalil Herbert, who rushed for 75 yards on 18 carries in last Sunday's win over the Raiders, is expected to make his first NFL start against the Packers.
Eiselen also signed with the Bears in 2020 as an undrafted free agent from Yale. He appeared in one game as a rookie, playing three snaps on special teams in a Week 9 loss to the Titans, and spent the first five weeks of this season on the practice squad.
For the second straight year due to COVID-19, NFL teams are permitted to flex up to two players from their practice squad to their active roster prior to 3 p.m. (CT) the day before a game without having to place any players on waivers.