The defender appeared to stamp on Chelsea star Eden Hazard as the Red Devils bowed out of the competition in a 1-0 defeat to the Premier League leaders.
United have since been slapped with a charge for failing to control their players in the aftermath of Ander Herrera's dismissal for picking up two yellow cards at Stamford Bridge.
However, Rojo was not hit with a punishment, leading to several people taking to Twitter to blast the FA's decision.
One of them was Bournemouth's Tyrone Mings, who was recently handed a five-game ban for stamping on Zlatan Ibrahimovic's head during a Premier League game between the two at the start of March.
Lol— Tyrone Mings (@OfficialTM_3) March 14, 2017
And several other Twitter users felt the need to follow in Mings' footsteps, though with slightly more characters!
Mings gets 5 match ban for stamp Rojo nothing escapes for the 3rd time this season the FA showing once again its totally biased & corrupt— andy (@citymanblue) March 14, 2017
So Mings gets a five-match ban while Rojo gets off scot-free for the third time this season? And United/Mourinho have the gall to complain?— Shagun Goel (@gshag_16) March 14, 2017
Mental how Mings gets 5 games for a stamp yet Rojo gets nothing— MΛC (@ryanmcii) March 14, 2017
Mings 5 games— Stan Collymore (@StanCollymore) March 14, 2017
Rojo 0 games
Same offence, time to bring retro punishment that supersedes refs original decision if necessary.
So Mings gets a 5 game ban for a clear accident while Rojo gets nothing for a blatant intentional stamp. But the FA are against United.— Joel Rabinowitz (@joel_archie) March 14, 2017
Suspect Bournemouth will be nonplussed as to how Rojo has escaped charge for alleged stamp on Hazard given Mings situation— James Ducker (@TelegraphDucker) March 14, 2017
Mings's is a stamp. Rojo's not a stamp. Small club justice v big club justice.— Oliver Holt (@OllieHolt22) March 14, 2017
So Marcos the butcher Rojo's stamp on Hazard has gone punished.— Kay (@Cesc4Pass) March 14, 2017
I bet Judas didn't see that one.
Once again the FA gone soft on Man Utd! No further action to be taken on Rojo for that stamp!— Karl Murch (@Karlos_Albertos) March 14, 2017