The problem plagued the 25-year-old for much of last season following his January move from Motherwell.
It has failed to improve over the summer and Rams boss Nigel Clough confirmed: "We're still in limbo with him. It's looking like he's probably going to need an operation.
"It's very frustrating because he has been in the hands of specialists and if they had said this last March or something he could have had it done and been fit by now.
"As it is he's going to be out for the next few months as well."
Porter is one of four Derby forwards struggling for fitness on the eve of the new Championship campaign along with last season's top scorer Rob Hulse (groin), Kris Commons (hamstring) and Luke Varney (flu).