The teenager picked up the injury against Ipswich last month, and was told he would need surgery to cure the cruciate ligament complaint.
Commons is disappointed at having to miss the rest of the season, but feels he will come back next term a stronger player.
The 19-year-old said: "Hopefully I will come back stronger and can look forward to next season.
"The op revealed cruciate ligament damage and that's going to need surgery, probably in around three weeks.
"I'll be on crutches for a week afterwards and then out for six months, which is pretty well the rest of the season.
"I'm gutted but I've just got to get on with it. I'm still young so time is on my side."
