Jackson had been sidelined by a right shoulder strain. Krol is coming back from a left oblique strain.
Atlanta also recalled left-handers Rex Brothers and Max Fried and infielder Rio Ruiz from Triple-A Gwinnett on Friday. Catcher Tony Sanchez also was promoted to the major league club, and infielder Micah Johnson was designated for assignment.
Active major league rosters can expand to as many as 40 players in September.