If Bell doesn't join the Steelers by Tuesday, he will be ineligible to play for the remainder of the 2018 season.
The Steelers placed a franchise tag worth $14.5 million on Bell during the offseason, and the sides couldn't agree on a long-term extension.
Bell is poised to leave that money on the table and sit out a full season before he hits free agency after 2018. While Pittsburgh can place the transition tag on Bell, which would give it the right to match any offer made to him, Bell likely won't stay with the Steelers.
The 26-year-old is a three-time Pro Bowler and two-time First Team All-Pro who led the NFL with 406 touches last season and finished with 1,946 yards from scrimmage and 11 touchdowns.
Although that type of production is difficult to replace, second-year running back James Conner has filled in admirably. Conner is enjoying a Pro Bowl-caliber campaign with 771 yards and 10 touchdowns on the ground to go with 39 receptions for 387 yards and another score.
Pittsburgh is once again one of the AFC's top teams even without Bell, as it leads the AFC North with a 6-2-1 record.
Conner suffered a concussion during Thursday's win over the Carolina Panthers, and his status for next week's game against the Jacksonville Jaguars is uncertain.
If Conner can't play next week and Bell doesn't return, veteran Stevan Ridley and rookie Jaylen Samuels will likely split the bulk of the backfield work.