Jurgen Klopp has admitted that he is unsure about the full extent of Joel Matip's latest injury problem after the Liverpool defender limped off against Borussia Dortmund.

Matip, who missed the final two months of last season with a thigh problem, lasted just 22 minutes of the Reds' 3-1 defeat in North Carolina.

The 26-year-old has not suffered a recurrence of that previous issue, which required surgery to rectify, but Klopp concedes that potentially losing one of his centre-backs is a big setback.

"It's not the same injury. It's another muscle - it looks like, hopefully, only a little tear," he told reporters after the match. "We will see, but we cannot say any more about it now.

"It is of course the most negative thing about the day. It's not cool. Things like this can happen, but you don't want them to. Maybe on Monday or Tuesday we will know more, but in this moment we only know that it was too painful for him to carry on."

Matip had previously featured in three of Liverpool's four summer friendlies and was confident that his injury problems were behind him.