The injury occurred Thursday night in the first period of the Thrashers' 5-3 victory against the New York Rangers, their first win of the new season.
"It just snapped," Lehtonen told the Atlanta Journal-Constitution. "It's OK to walk and do desk work, but not to be a goalie right now."
The same injury plagued the five-year veteran during training camp.
Before the latest injury, Lehtonen struggled with a 4.49 goals-against average, as Atlanta failed to win any of its first five games.
The start led to the firing of Coach Bob Hartley Wednesday.