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 team-mates this week after completing his rehabilitation.
However, ahead of Tuesday's Coppa Italia semi-final 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 due to an Achilles issue.
Meanwhile, Ante Rebic and Nenad Tomovic will continue to recover from their respective injuries in the week.