The likes of Mats Hummels, Neven Subotic and Ilkay Gundogan have been among those sidelined for Dortmund this season, with only the latter nearing an imminent return.
Klopp's injury list has contributed to a slump in form domestically, with his side winning only one of their last six Bundesliga matches to lie fourth over the winter break.
While Klopp is pleased to have been able to blood younger players, he has bemoaned the absences of several senior players.
"As far as injuries, it is the hardest (season) of my coaching career," he told The Rheinische Post.
"The good thing is that Erik Durm, Marian Sarr, Jonas Hofmann and others get the chance to play in a top league.
"A complete axle is broken with us; that changes the whole game."
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