Mannelly, the Bears all-time leader in games player with 239, left Lambeau Field Monday night in a walking boot and is considered "week-to-week" and could be out "a couple weeks", according to Bears head coach Marc Trestman.
"It doesn't look good for this week," Trestman said.
Cain, 33, originally entered the NFL with the Bears in 2004, but spent the last four seasons as the long snapper for the Jacksonville Jaguars (2009-2012) before losing his job this preseason. Cain worked with current Bears special teams coordinator Joe DeCamillis and punter Adam Podlesh in Jacksonville.
Bears general manager Phil Emery described Mannelly, a team captain and locker room leader, as “one of the eight wonders of the world” at a recent press conference. Mannelly has missed only nine games over his decorated career in Chicago.
The Bears also made their signing of veteran defensive tackle Jay Ratliff official on Wednesday.