The FA is reviewing an incident during Newcastle's clash with Manchester United at St James' Park when Jonny Evans and Papiss Cisse appeared to spit at each other.
United defender Evans and Newcastle striker Cisse squared up off the ball five minutes before half-time in Wednesday's Premier League fixture.
Evans appeared to launch spit at Cisse as the Senegal international rose from the turf after the pair tussled for the ball with the game goalless.
Cisse stretched out to grab his rival in response and seemed to aim saliva back in Evans' direction.
The FA is now awaiting the referee's report and will review match footage in relation to the incident.
Neither Manchester United manager Louis van Gaal nor opposite number John Carver saw the incident, which happened behind referee Anthony Taylor's back and seems certain to be examined by Football Association disciplinary chiefs.
But the Dutchman - whose side went on to win through a late Ashley Young strike - said he "could not imagine" Evans being involved in such an incident.
"It is too far from the bench to see someone spitting," Van Gaal said.
"I can't judge it but I can't imagine Jonny Evans - because he is a very modest person - would do that."
Newcastle boss Carver said: "I haven't seen the incident. I just saw a fracas on the halfway line between the two players.
"As far as I'm concerned I need to have a look at it."
Should the pair be found guilty, Sky Sports understands the bans will be lengthy following a change to the rules at the beginning of the season.
Both could be facing a six-match suspension, after the FA fell into line with FIFA which demands that term for such an offence.
However, Cisse could be forced to sit out seven matches having already been suspended by the FA for violent conduct earlier in the season.
Written by Sky Sports