Napoli striker has undergone an MRI scan that has revealed he has torn his anterior cruciate ligament in a devastating blow to both club and player.
The Polish striker was forced out of the game against SPAL after 70 minutes, and will now undergo surgery on Monday.
It is a huge shame for Milik, who suffered a similar injury last season which sidelined him for nearly six months and 21 games of the season.
Thankfully, his injury this time is to his other knee, and his expected recovery time at this point is belived to be around the five month mark.
Napoli will be hoping to continue their fine start to the season in Milik's absence - they have won all six matches in Serie A so far, and many believe that under Maurizio Sarri, they have a real chance of challenging Juventus for the title this season.
The injury to Milik may mean Sarri has to assess the transfer market in January, with the club competing on four fronts this season without another out-and-out striker.