The Germany striker is recovering from a knee injury.
"The treatment he underwent in Munich last week certainly helped to reduce the pain, but it has not entirely gone yet," chief medic Paolo Manetti told Viola Channel.
"Therefore the final passage of returning to the field is not yet possible. The inflammation is of a tendon that passes over the collateral ligament that suffered the lesion a while ago.
"We are doing all that we can, including pain-killing injections that necessitate rest. Trying to figure out which game he could return in, whether against Bologna or Sassuolo, unfortunately will get us nowhere, because first we must resolve this problem.
"At the moment he is not expected to return to action soon, because first we must help the lad work without feeling pain. Only then will he return to training with the squad and then Vincenzo Montella will decide when to play him.
"We are close to the complete solution of the problem, but it is first important to wait for the full recovery of this inflamed tendon."
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