The full-back was cautioned for an incident five minutes into the second half as the Light Blues prepared to defend a Celtic corner.
TV footage showed Flanagan appearing to throw an arm into Brown’s face, causing the Hoops skipper to fall to the floor. Match referee Kevin Clancy had a clear, unobstructed view of the incident and chose to book the Rangers player.
Iain Crocker, commentating on the game for Sky Sports, said after the flashpoint: “When [the incident] actually takes place, the ball isn’t in play.
“I think you can see Kevin Clancy has seen something - he maybe hasn’t seen the severity of it and that’s why I think he’s shown a yellow card, and [not given a] penalty.”
Summarising on BBC Sportsound, Jim Duffy said Flanagan had been a “very lucky boy”, adding: “You just can’t do that. Jon Flanagan [has] put a forearm into Scott Brown’s face.
“[Rangers boss] Steven Gerrard has been talking about the discipline of his players. He’s a very lucky boy not to be sent off.”