Last season's top scorer, who had surgery on the problem, has started running unhindered and has been doing catch-up double training sessions at the Academy of Light training ground in a bid to get fit.
Fletcher is unlikely to feature in Fulham's visit to the Stadium of Light for the season's opener this weekend.
Sunderland boss Paolo Di Canio said: "There is a big improvement and we are happy with his progress. We are waiting for him."
For the big stories and the banter follow us on Twitter: @tribalfootball