Saiz was dismissed in stoppage-time for spitting at Newport County's Robbie Willmott as Leeds were knocked out of the FA Cup by the League Two side on Sunday.
"Samuel's behaviour was unacceptable and the player will now be subject to an internal disciplinary hearing on top of serving a six-game ban", read a Leeds statement.
"I would like to take this opportunity to sincerely apologise to the players, staff and supporters of Leeds United for my behaviour in yesterday's game at Newport County", Saiz said.
But the FA have now confirmed the allegations of spitting and say the attacker will be given an automatic six match ban.
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TV cameras failed to pick up the incident which followed Newport's injury-time victor, but it is believed Saiz spat in the direction of Robbie Willmott which sparked an angry reaction from the home players.
Saiz was previously accused of spitting by then Port Vale boss Michael Brown in August but was cleared of wrongdoing.
"I accept the punishment handed to me by the FA".
Saiz, 26. will be sidelined until the Sky Bet Championship fixture at Derby on February 20, missing six league games as Leeds bid to maintain their play-off challenge.