Calling him his 'top performer' to date this season, the Forest boss confirmed on Wednesday that Cohen would be out for up to a year with knee ligament damage sustained in the game against Derby.
He told BBC Radio Nottingham: "I was distraught when it happened and distraught when I found out the results.
"He has to take nearly a year out but he'll be back and he'll be stronger and better, but we'll miss him."
QPR defender Clint Hill has been drafted in as cover.