Callahan, who is entering his eighth season, has spent the last three years in Denver after playing in Chicago from 2015-18. Chargers coach Brandon Staley was on the Bears staff in 2018 and then went to the Broncos in 2019 when Vic Fangio became coach.
The 5-foot-9, 188-pound cornerback was plagued by injuries in Denver. He missed the 2019 season due to foot issues and then played only six games last year due to a knee injury.
Callahan is likely to be the slot cornerback in nickel situations. He would replace Chris Harris Jr., who has not been re-signed. Coincidentally, Callahan also took over from Harris in Denver in 2019.
In 66 career games, the 30-year-old former Rice player has six interceptions, 29 passes defensed, 193 tackles and five sacks.