Viduka played for 45 minutes in Leeds' 1-0 defeat to relegation rivals Spurs, but did not come out after the break.
The Aussie's absence at St Mary's means Leeds' problems are mounting, with them now five points adrift of Premiership safety.
Gray will also be without skipper Dominic Matteo for the game against The Saints through suspension, while Salomon Olembe and Lamine Sakho, who both played some part today, will be away on international duty in the African Cup of Nations.
Gray said: "Mark has a personal problem back home with one of his family who is ill.
"He has been up all night for the last two nights and he almost flew back on Friday night.
"But he decided to give it a go, however it was clear his mind was not really on it.
"I had a chat with him at half time and now he's going to go back home."