Walden was in the bullpen and available to pitch against Colorado.
In a corresponding move, Atlanta optioned lefty Alex Wood to Triple-A Gwinnett.
Walden missed 32 games with a strained left hamstring. He pitched one scoreless inning in a rehab outing for Gwinnett on Sunday.
He posted a 2.92 ERA in 14 appearances before going on the disabled list May 5.
Wood was 5-6 with a 3.43 ERA in 18 appearances and six starts for Atlanta.