Raphael Guerreiro is set for a "lengthy spell on the sidelines" after undergoing a foot operation on Monday, Borussia Dortmund have announced.
Guerreiro, 23, suffered the injury while on Confederations Cup duty for Portugal last month, and the Portuguese FA told BVB after initial tests that he "had only sustained a bruised foot."
However, the Bundesliga side said in a statement that further examinations in Dortmund then "revealed that the foot of the Portugal international has suffered a clear break," adding that the left-back "will be out until October/November following the surgery."
Guerreiro's long-term injury hands another huge blow to new Borussia Dortmund head coach Peter Bosz, who already has to do without Germany internationals Marco Reus and Julian Weigl.
While the latter could return early on in the season following an ankle fracture sustained late last season, winger Reus is not expected back before 2018 after undergoing anterior cruciate ligament surgery earlier this summer.