The Rossoneri had only just signed the German on loan from Genoa to provide cover in midfield following injuries to Gennaro Gattuso and Alberto Aquilani
The German midfielder arrived at San Siro on loan from Genoa earlier this month to provide cover for injured duo Gennaro Gattuso and Alberto Aquilani.
However, the Rossoneri have announced on Sunday evening that Merkel had suffered medial ligament damage to his right knee and that he will be out of action for "eight weeks - unless there are complications."
The 19-year-old Germany Under-20 international sustained the injury during the Coppa Italia quarterfinal victory over Lazio at San Siro on Thursday night.
Merkel, who is co-owned by Genoa, rejoined the Rossoneri on a six-month loan on Jan. 17.