Wolfsburg's Josuha Guilavogui broke his neck in Saturday's pre-season loss to Sporting CP, the Bundesliga club have confirmed.
The midfielder, who joined permanently in May after a loan spell from Atletico Madrid, suffered the blow following a challenge in the second half of the 2-1 defeat in Lisbon.
He was taken to hospital and an MRI scan revealed a broken cervical vertebra, though the club put no estimate on his return date.
Guilavogui told the club's official website: "Now I hope the healing process goes positively and that I recover as quickly as possible, so that I am available to the team again."
Dieter Hecking's men finished eighth last season and kick off their new Bundesliga campaign at Augsburg on August 27.