The Scotland international has been blighted by a chronic bowel condition for the past two years and made his first appearance since Boxing Day 2012 on Sunday. Fletcher appeared for the last 20 minutes at Villa Park as a replacement for Ryan Giggs, as United recorded a comfortable 3-0 triumph.
The 29-year-old is hopeful his problems, which forced him to take a prolonged break from the game, are now behind him.
Speaking to MUTV, Fletcher said: "This is it — I'm back for good. This is hopefully the game which means I'm back now. I seem to have come through the setbacks and health issues and I'm thankful for that. It's onwards and upwards now.
"I always believed I would come back; I kept that mindset. I think other people around me were trying to make me think otherwise, but I stayed strong and believed I would get back. Ever since I was a kid, I've wanted to play football. I've been fortunate enough to have the opportunity to play for Manchester United and I wasn't going to give that up lightly.
"I battled through it, and I'm only 29 years old. I think I've got plenty more years of playing left in me. I could have a career all over again if I get my health right, and hopefully that's what I'm going to do. I'm still building up my fitness, but I don't want to take it easy. I've done enough of that, and I want to be involved, to get us back to where I know we should be."