Kiessling crocked with torn leg ligament
Kiessling was stretchered off in the 12th minute of a 0-0 draw against Nuremberg on Sunday after clashing with an opponent.
He underwent further tests on Monday before his left leg was put in plaster, the club said. It was not immediately clear how long the 26-year-old forward would be out of action.
Germany host Turkey on October 8 and travel to Kazakhstan four days later.
"Injury bad luck has hit us hard early in the season," Leverkusen coach Jupp Heynckes said in a statement. "We must now find ways to deal with it."
There was some good news, however, with fellow striker Patrick Helmes, who also limped off with a thigh muscle injury, likely to miss just one match, the midweek clash against Eintracht Frankfurt.