Liverpool were angry that Manchester City midfielder Rodri did not get booked for his 34th-minute trip on Cody Gakpo
A number of players protested that City midfielder Rodri should have had a second yellow card - and so been sent off - for a trip on Cody Gakpo.
Spaniard Rodri had just been booked for pulling back Diogo Jota.
Liverpool have until next Wednesday to respond.
Referee Hooper indicated to Rodri when showing him the yellow card following the Jota foul that his caution was for an accumulation of offences.
Liverpool took the lead early in the first half against City at Etihad Stadium but were beaten 4-1.
-- Courtesy of BBC Sport