The 50-year-old spent four-plus seasons at the helm with the Oakland Athletics beginning in 2007, racking up a 334-376 record before he was fired on June 9.
"Bob Geren's experience as a former manager and bench coach will be a great fit for us," said Collins. "Adding a former major league catcher like Bob to our staff will be ideal."
Prior to being named manager of the A's, Geren spent 2003-06 on the club's coaching staff as bullpen coach, then bench coach for Ken Macha.
The native of San Diego played five seasons with the Yankees and Padres from 1988-93.