The Scotland international has played just 13 games since announcing he was taking an extended break from football in November 2011 after he was diagnosed with ulcerative colitis.
Fletcher has undergone numerous operations - the last of which took place in January - and been put on a strict dietary regime in a bid to correct his stomach condition.
But the 29-year-old will miss the start of the season, and Fletcher admits he has no firm date pencilled in for his comeback.
Fletcher said: "I'm still in the hands of the surgeons at the moment - I've not actually started training yet.
"I've been doing my own little bit in the gym, which is keeping me ticking along nicely and I'm waiting for the surgeon to give me the go-ahead to join in training.
"I don't have a timescale for when I will be back playing really. Hopefully it is not going to be too far away in the distant future.
"I just need to be patient. After all I've gone through I think it's important that I'm patient. If it means a few extra weeks to get myself right, then so be it."