Borussia Dortmund star Neven Subotic is set to miss up to six weeks with a calf injury, the club announced on Tuesday evening.
The Serbia international picked up the problem during the first session at Dortmund's winter training camp in La Manga, Spain.
The 24-year-old had originally thought he would be able to return in time for the resumption of the Bundesliga campaign on January 19 - telling reporters on Monday "I feel much better already" - but an MRI scan on Tuesday confirmed a serious muscle tear in his calf.
"He will be out for about six weeks," a club statement said. "This means he will miss the first Bundesliga matches at the start of the year."
That expected absence would see Subotic miss as many as six matches, including the Bundesliga trip to Bayer Leverkusen and the first leg of their Champions League last-16 tie away to Shakhtar Donetsk.
The Subotic injury leaves Dortmund with only two fit centre-backs, Germany international Mats Hummels and Brazilian Felipe Santana, who is believed to be leaving the club at the end of the season after failing to win a starting place in Klopp's team.
The German champions will now most likely promote Germany Under-19 international Koray Gunter, 18, from the club's reserves. Gunter has featured in 13 Bundesliga III games this season and was part of the BVB squad in their Champions League match against Manchester City back in December 2012.
Local papers claim Dortmund could now also consider signing an experienced centre-back during the winter transfer period, with the club yet to comment on the Subotic injury and its consequences.
However, general manager Michael Zorc told Ruhr Nachrichten: "We are not considering buying someone for that position."