Cook, the No. 34 overall pick of the 2010 draft by the Minnesota Vikings, appeared in six games for the 49ers last season and had three tackles, a pass defensed and two special teams tackles. A hamstring injury ended his season before the Niners re-signed him to a one-year deal in March.
In 40 career games, 29 starts, Cook did not have an interception and, per Pro Football Focus, he gave up a league-high nine touchdowns in 2013 for the Vikings.
Tramaine Brock and Shareece Wright are the starting corners with Dontae Johnson, Keith Reaser, Kenneth Acker the backups (Johnson has come on in nickel defense with Brock sliding into the slot) and Jimmie Ward playing a hybrid DB role.
The six DBs have a combined nine interceptions, with Brock accounting for seven of them.
With the move, the Niners now have one open spot on their 90-man roster.