The Scotland international spent a combined total of nearly two years out of action after being diagnosed with ulcerative colitis, but has fought back to rebuild his career after a run of games under David Moyes.
Fletcher admitted that he had been "blase" about the condition at first and had lied to the club about the severity of the problem to avoid embarrassment.
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"I was a professional footballer at Manchester United. I felt untouchable.
"I started my battle with it in 2010 and ultimately had to have surgery so I could continue my career but I stayed silent about it until 2011.
"Other than the manager [Sir Alex Ferguson], nobody at the club knew. I found it difficult making up stories about why I wasn't at training, looking ill and rushing off to the bathroom.
"I was lying to people. Once I started talking about it, it was such a relief."
Fletcher made his first-team return against Aston Villa in mid-December and has gone on to make eight appearances, starting United's Premier League wins against Hull City and Swansea City in January.