The Scotland international was diagnosed with the chronic condition back in 2011, but was only able to return to football after undergoing surgery last year.
"You can be running to the toilet 10, 20, 30 times a day and losing a lot of blood," BBC Sport quotes Fletcher as saying. "I ended up in hospital a couple of times on an IV drip. Surgery enabled me to be here today. I'm very fortunate."
Fletcher revealed that he began to experience symptoms in 2008, but remained silent about the affliction until after his diagnosis several years later.
Following the successful course of treatment, the 30-year-old is confident that he can keep his illness under control by sticking to a strict health and fitness routine.
"The surgery was a success, I'm better now - I'm living a normal life and back playing football," he added.
"I can't have any relapses now. I'm not going to have episodes where I get extremely ill, get taken to hospital or have a breakdown at any moment. I've got a routine that I stick to and it's working fine.
"There are no guarantees with surgery, but it has enabled me to be back playing for Manchester United and feeling great."
Fletcher recently joined forces with fellow ulcerative colitis sufferer and rugby legend Lewis Moody to launch United for Colitis, a new fundraising initiative for the Crohn's and Colitis UK charity.