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.

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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.