The Germany international picked up the issue in September's 1-1 draw with Cagliari and has suffered setbacks in his return to fitness.
Having only made three Serie A appearances since joining from Bayern Munich, the 28-year-old will return to training with his teammates this week after completing his rehabilitation.
However, ahead of Tuesday's Coppa Italia semifinal first leg against Udinese, Vincenzo Montella has been hit by injuries to Alberto Aquilani and Massimo Ambrosini.
Aquilani suffered a hamstring injury in Saturday's defeat to Cagliari while Ambrosini will see a specialist because of an Achilles issue.
Meanwhile, Ante Rebic and Nenad Tomovic will continue to recover from their respective injuries in the week.