The 23-year-old last appeared in a 3-3 friendly draw for Germany against Paraguay in August 2013.
Since then, midfielder Gundogan has been out with a back injury, and on Tuesday Klopp moved to deny reports that he was set for a return in Wednesday's Bundesliga clash at Signal-Iduna Park.
"The day Ilkay Gundogan will return to action will be a great day for everyone," he said.
"But 14 months on the sidelines are 14 months. We will stay calm, in spite of our injury problems."
Klopp is expected to have Mats Hummels, Neven Subotic and Ji Dong-won available, although Henrikh Mkhitaryan is absent after picking up a foot injury in Saturday's 2-0 defeat at Mainz.
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