His main injury was a broken fifth metatarsal bone in his left foot. that kept him out of playing till the end of the season but some time after the operation a periostitis occured there and this is the condition he was suffering from during the preseason.
That's an inflammation of periosteum, the outer layer of the bone:
It can be caused by infection or be a non-infective complication of an injury. I don't know what was the cause in Sissoko's case but I can imagine why he was sidelined for such a long time: he probably barely could walk without pain.
The fixture list coming into the Bayern game is horrifically crammed. Come then either we'll have to field some tired ass guys or a 2nd string midfield.
I think we might have to see guys like DC in midfield to make the numbers balance