"Definitely they are out," Adelaide coach Aurelio Vidmar said Wednesday.
He said Cassio has had a "high fever" since Friday and struggled during practice this week. "He wasn't looking good, so we don't take any risk," said Vidmar.
Diego was out due to a tight groin, he said. "We thought he would be all right for tomorrow night. But it's still a little bit sore. We really don't want to take a risk," said Vidmar, adding that Diego would hopefully recover in time for the next game.
Vidmar appeared confident without the Brazilian duo, saying: "Strong players will come into the squad. We expect them to do their job. For sure, it's probably not our strongest line up, but we've got confidence."
A 6-1 win over the Wellington Phoenix on Friday lifted the Reds to the top of the A-League.
A win over Waitakere would give the Australian club a chance to avenge Gamba Osaka of Japan for their defeat in the AFC Champions League final, with giants Manchester United awaiting the winners of that showdown in the semi-finals.