"Alex Manninger stopped yesterday during training, feeling pain in his left calf," explained the statement.
"For this reason, in the morning, the player underwent an examination that showed the presence of a first-degree lesion to the medial muscle of his left calf. The player will resume altheltic activities in approximately 20 days."
Last season the veteran made 11 appearances in Serie A. The 33-year-old is not the only worry in goal for the Bianconeri, as first-choice Gianluigi Buffon is out with a long-term back injury after having an operation.
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