Romania defender Rat was shown a straight red card in Saturday's 3-0 La Liga win over Celta Vigo for an off-the-ball clash with Brazilian forward Charles.
And the club confirmed on Wednesday that the former West Ham full-back will now sit out the next four games.
Coach Paco Jemez will also be without on-loan Valencia man Viera for a minimum of four weeks due to a hamstring strain.
Viera has played 28 times for Rayo in all competitions since his temporary switch in September and the Madrid club will be hoping his substitute appearance against Celta is not his last before he returns to his parent club.
