The Daily Mail says Fletcher is one bad tackle away from ankle surgery, it has emerged.
The Scotland international is recognised as United's most important midfielder and returned to action after an injury lay-off as his team drew 3-3 with CSKA Moscow at Old Trafford on Tuesday.
Fletcher said last night: "I injured my ankle in the Sunderland game last month. A bit of bone has broken off the back of my ankle and is stuck, so every time I bend it is causing a bit of pain.
"I have had an injection to clear it up but eventually it is going to need to be cleaned out. There has been a build-up of fluid as well. I will have to get something done but the injection will see me through a few months.
"It should be OK to the end of the season. That is the plan - to get me through to the summer.
"Striking a ball or bending the ankle in a flexed position or pushing off hurts. It got to the point where it was too sore to train and play so something had to be done and I had the injection."