Veteran Brazilian left-back Cassio was brought into Adelaide's starting XI last Saturday to cover Zullo's absence and impressed head coach Josep Gombau in the Reds' 1-1 draw away to Perth Glory.
It was Cassio's first start of the A-League season after his preseason was interrupted by a quadriceps injury.
But the 33-year-old did so well in Adelaide's draw - their first point in four matches - that Gombau is considering leaving him in the line-up to play Melbourne Victory on Saturday night at Zullo's expense.
"He (Cassio) did very well and this is important because he's a player that, for us, needs to be important," Gombau said on Friday.
"He played better than we expected. Not (based on) his quality, which is very high, but for his performance because for more than five months he did not play an official game.
"I think when one player is doing well you need to keep him."
Zullo has started his loan spell with the Reds from Dutch club FC Utrecht in lively fashion, which saw him called up to the Socceroos squad to take on Costa Rica.
But with Adelaide having kept Glory to just four shots last week, Gombau is reluctant to tinker with his defence despite having Zullo and Nigel Boogaard, who missed the game through suspension, available again.
Adelaide have the equal-worst defensive record in the A-League ahead of their trip to Melbourne and have yet to keep a clean sheet this season.
"I am satisfied with the first 11 that played last week. Maybe we put one or two in, maybe, maybe not," Gombau said.
The Reds took the lead against Perth through Osama Malik early in the second half but had to settle for a draw when the same player left Glory forward Chris Harold alone to score with 21 minutes remaining.
But Gombau made it clear Malik's starting position is absolutely secure.
"To Malik, I say my confidence with him is unlimited," the 37-year-old Spanish coach said.
"He can make any mistake, he's a very good player, I have confidence with him and he will play."