The Detroit Lions have in fact signed defensive back Don Carey. Vincent Fuller was not released or placed on injured reserve to make room for him on the roster, however. It turns out the Lions instead parted ways with tight end Joe Jon Finley to make room for Carey.
Finley was first let go by the Lions during the final round of roster cuts. He was brought back before the 49ers game because Tony Scheffler was out with a concussion. With Scheffler back, Finley is no longer needed and is once again a free agent.
In addition to the change to the 53-man roster, the Lions also announced a change to their practice squad. Fullback Matt Clapp has been released less than a week after being re-signed, and tight end Nathan Overbay has been re-signed less than a week after being let go.
Because the Lions want an extra tight end in case another injury happens, they decided to add Overbay back to the practice squad with Finley being let go. (Finley is not eligible to be on the practice squad.)