Team officials say he is expected to miss six weeks.
McEachern played in at least 77 games in four of the past five seasons, but has been hampered by a groin injury and disk problem this season. He has 10 goals and 16 assists in 46 games.
McEachern, who turns 34 on Feb. 28, has scored just one goal since Nov. 28 for the rejuvenated Thrashers, who are 7-3-1 since Bob Hartley took over as coach.
In 801 career games, McEachern has collected 237 goals and 279 assists with stops in Boston, Los Angeles, Pittsburgh, and Ottawa.
In another move, Atlanta assigned defenseman Mike Weaver to Chicago of the American Hockey League. The 24-year-old Weaver has five assists and 20 penalty minutes in 40 games.