"It was a twist injury in the right knee," Lienen told Bild newspaper. "He was in real pain."
Kicker reported on Tuesday that Cooper's injury was diagnosed as medial collateral ligament (MCL) damage, and that the 25-year-old might be sidelined for only three weeks.
Cooper, who played 11 minutes in the USA's final World Cup qualifying game, a 2-2 tie against Costa Rica Oct. 14, joined 1860 Munich last summer from FC Dallas. He scored twice in his first two league games and notched a goal in the German Cup. Cooper then went scoreless in five straight starts and was used as a late sub in the last four league games, having lost his starting spot to 18-year-old Peniel Mlapa.