The 29-year-old has been battling ulcerative colitis for the past five years and underwent surgery in January as part of a "planned procedure".
Asked on BBC Radio Five when he would be back in action, Fletcher replied: "Hopefully, not too long after the start of the season. I think I will just miss the beginning of the season."
The Scotland captain took nine months out to see if his symptoms settled down before returning last September, and seemed to be making rapid progress.
However, after appearing in three games out of four at the end of November, he was restricted to two very brief substitute appearances - the last of which came in the 4-3 win over Newcastle at Old Trafford on Boxing Day.
"I've had it for five years but really struggled to control it for the last three years," he said. "It's had a big impact on my day-to-day life, never mind trying to play football.
"Medication wasn't helping me but surgery was the only solution if I wanted to have not only a future playing football but even a quality of life going forward.
Fletcher added: "It's all going well. I am still under the hands of the medical team just now, and the surgeon who is looking after me. Everything is going to plan so far and I am not that far away now.
"I am doing exercise on my own. I've not been given the green light to join in contact in football training but I am doing my own running and physical activities and feel great.
"Hopefully, in the next few weeks I am going to get the green light to join in and to start making the comeback, which is looking very good."