Jurgen Klopp described the Brazilian's injury as a "catastrophe for us" - which could be bad news for Liverpool should he miss any of their upcoming fixtures.
Liverpool were frustratingly held to just a point, although they regain top spot over title rivals Manchester City.
According to the Mail's Dominic King, Firmino did not have his foot in a protective boot, but left on crutches whilst departing Old Trafford.
Despite labelling Firmino's injury as a "catastrophe", Klopp said at his post-match press conference: “Unfortunately, I have no clue. It was something with the ankle, that's for sure.
“Bobby said at first he could carry on but then he showed it didn't work.
“I don't know exactly; it is Bobby, so it will be quicker than with all the others but first we have to know what it is exactly – and we don't know at the moment.”
The hope will be it's just a minor strain rather than any ligament damage, which would be a big blow for the Reds.
The club are expected to provide another update on his injury in the coming days.