Fletcher was incensed when Matthijs De Ligt was forced off the field due to blood gushing from his head.
United conceded from a corner while he was off the pitch in an eventual 2-1 win over Brentford.
As reported by the Daily Mail, fourth official Gavin Ward told the Commission: "Following the half-time whistle and as I made my way up the tunnel, I was approached by Darren Fletcher of Manchester United in an extremely aggressive manner pointing and shouting at me, 'You are all f***ing s***, that is a f***ing joke, every f***ing week.’
"I asked him to calm down and highlighted that his behavior was completely unacceptable at this point.
"He then needed to be held back by another member of staff, as he tried to get closer to me.
"With this, I turned away and continued to make my way to the match officials changing room.
"Once in the changing room and when the other match officials arrived, I made them aware of the conduct of Mr Fletcher."