A source familiar with the situation told ESPNChicago.com's Michael C. Wright that Rachal became upset after he was told he was being replaced in the starting lineup by Chris Spencer for Sunday's game against the Minnesota Vikings.
According to league rules, Rachal has five days to return to the team or he won't be able to return this season.
Rachal was part of a unit that yielded six sacks in a 32-7 loss to the San Francisco 49ers on Monday night.
Rachal started eight games for the Bears at left guard this season after he replaced Spencer in Week 3.
A five-year NFL veteran, Rachal spent the previous four seasons with the 49ers, with whom he started 38 games.
The Bears elevated rookie guard/tackle James Brown from the practice squad to fill Rachal's roster spot. Brown was a rookie free agent out of Troy who played well at times in the preseason but failed to make the initial 53-man roster.