The 24-year-old's fitness woes means he has made just 11 appearances for the Championship outfit so far this campaign.
Johnson has revealed that even the specialists are unsure as to when he will be able to return to full fitness.
He grudgingly accepts that he may have to sit out the rest of the campaign in order to completely recover from the problem.
"It is slowly getting better," he told Sky Sports. "But it is not easy as it is in a difficult position between the tibia and the ankle.
"I need to take time to let it heal and get a lot of rest in so that it doesn't happen again in the near future. Everyone has an opinion on it, so we are not 100% sure how long it will take for it heal.
"I hope that I will be back available for first-team action before the end of the current campaign but at the moment I'm not sure whether that will be possible.
"For me, it is most important to come back strongly from my injury, have a good summer and then return to the form I know I am capable of next season."