Saiz was sent off in injury time for spitting at Newport midfielder Robbie Willmott, as Leeds suffered a shock 2-1 third-round defeat to Sky Bet League Two opposition.
Referee Mike Dean confirmed later that he showed Saiz the red card for spitting, and Leeds could now lose the influential Spaniard from their Championship promotion bid until the end of February.
UEFA's rulebook punishes spitting with a mandatory six-game ban, and Jonny Evans and Papiss Cisse were both suspended after being found guilty of the offence during a Premier League match between Newcastle and Manchester United in March 2015.
Told of the spitting allegation, Leeds manager Thomas Christiansen replied: "If that's right why he got the red card then it's a problem.
"This is something we need to talk about. We need to speak and we'll see what happens."
It is not the first time that Saiz has been involved in a spitting row since joining Leeds from Huesca last summer.
Port Vale manager Michael Brown said Saiz spat at his defender Joe Davis during a Carabao Cup tie in August, but the Football Association did not bring any charges against the 26-year-old.
Christiansen made nine changes from the side which drew against Nottingham Forest on New Year's Day for the trip to south Wales.
But the Dane defended his team selection after Leeds lost to opponents 53 places below them on the league ladder.
"We were not intense enough and unfortunately we are out," Christiansen said.
"If you look, every player we had in the first XI today is from the first team or has possibilities in the first XI," Christiansen said.
"For this we expect more and it should be enough to pass to the next round.
"Against Burnley (in the Carabao Cup) I also made 10 changes and we had the perfect performance.
"It was the same against Leicester, we also made nine or 10 changes and did very well.
"So why not today? Why is it different?"
Gaetano Berardi's 30-yarder gave Leeds an early lead, with the Swiss defender's effort possibly brushing the heel of Pierre-Michel Lasogga on the way through.
But a Conor Shaughnessy own goal and Shawn McCoulsky's towering header in the final minute saw Newport deservedly into the fourth round for the first time in 39 years.
"Every single player should get the credit," said Newport manager Mike Flynn, who oversaw a remarkable revival last season to keep the Exiles in the Football League.
"This is much better (than last season), beating Championship teams like Leeds.
"I don't want to be winning relegation battles, I want to be winning FA Cup ties and climbing the table."