The fly-half was initially shown a yellow card for a high tackle to the head of Wales replacement Taine Basham in the final quarter.
But under new rules that allow a review panel to consider an act of foul play while the game is in progress, that was soon upgraded to a red.
England have two more warm-up games, against Ireland and Fiji.
But the worry for Steve Borthwick's side is that any ban could rule talisman Farrell out of the start of the World Cup proper, with England launching their Pool D campaign against Argentina in Marseille on September 9.
When Farrell left the field on Saturday, it meant England were down to 12 men with replacement prop Ellis Genge and full-back Freddie Steward having been shown yellow cards as well.
But England overhauled a 17-9 deficit thanks to Maro Itoje's converted try from a driving maul and a late penalty from replacement George Ford.
