Cook, 28, appeared in six games for San Francisco in 2014 before suffering a season-ending hamstring injury that required surgery. He was re-signed in the offseason to a one-year deal.
Cook was working with the second-team defense at left cornerback in training camp, competing with a number of younger players for playing time. The 49ers had two corners who missed their rookie seasons last year with injuries, Kenneth Acker and Keith Reaser, that are healthy this summer and competing for roster spots.
Including second-year player Jimmie Ward, the team's nickel cornerback, Cook was competing with nine players at the position. The starters have been Tramaine Brock and Shareece Wright, with Dontae Johnson playing in nickel situations.
San Francisco could use Cook's vacated roster spot to address positions dealing with injuries, or add help to the offensive line.
The 49ers are dealing with injuries at inside linebacker and receiver. Linebacker Michael Wilhoite, who started 16 games for San Francisco last season, is working his way back from an injury after being place on the non-football injury list at the start of camp. Fellow linebacker Nick Bellore has been dealing with an undisclosed illness.
Monday, wide receiver Bruce Ellington sustained a muscle strain, according to head coach Jim Tomsula, and did not practice Tuesday. Undrafted free agent receiver Isaac Blakeney is also dealing with an undisclosed injury.
But the story of through the first four days of training camp has been the offensive line, which has struggled while fielding new starters at four of the five positions. The only player earmarked to play the same position as last season is Pro-Bowl left tackle Joe Staley.
After losing former Pro-Bowl left guard Mike Iupati to the Arizona Cardinals in free agency and right tackle Anthony Davis to retirement, the 49ers are working Alex Boone at left guard, Joe Looney at center, Marcus Martin at right guard, and Erik Pears at right tackle.
That unit has not performed well against the 49ers deep and talented defensive front in team drills. Meanwhile, former Philadelphia Eagles guard Evan Mathis, 33, remains available on the free agent market after his release in the offseason.