The Sky Blues are in need of a central defender after rising Socceroos star Hayden Matthews sealed a move to English Championship club Portsmouth.
Alex Popovic was the next cab off the rank after Matthews in their 2-0 loss to Victory, but the centre-back is nearing the end of his loan spell from Gwangju FC.
With ample time left in the January transfer window set to close on February 12, coach UfukTalay was confident Popovic would remain at Sydney and hinted at a looming signing.
Matthews' replacement will have to be an Australian player, with the Sky Blues having used all their five visa spots.
"We'll have someone locked in - very close to that," Talay said on Thursday.
"He won't be available for the game on Saturday, but hopefully available for the game next week (against Western Sydney)
"We want to bring someone with experience, is a good leader as well, and I think he covers those bases and also can actually play the game.
"For us, he's a left-footed player. We need a left-side central defender."
Facing second-placed Adelaide at their Allianz Stadium home on Saturday, Sydney could call upon the services of crafty winger Costa if need be.
The high-profile off-season signing has been missing from action since their round 10 win over Melbourne Victory after re-injuring his hamstring.
Costa was originally slated to return for their AFC Champions League 2 match on February 12 against Bangkok United.
"He's tracking well. He joined the team this week (in training)," Talay said.
"He's still not 100 per cent but we'll see whether we utilise him this weekend, put him on the bench and hopefully have him available for the Wanderers game."
Coming off a bye after stunning Sydney for their first win of the season, Brisbane Roar will look for their second when they host Western Sydney on Friday.
On Saturday, league leaders Auckland will play a new-look Macarthur FC side without breakout star Jed Drew before Central Coast go toe-to-toe with rivals Newcastle Jets in another chapter of the F3 derby.
Perth Glory play Melbourne Victory in the final game of the round also on Saturday, with Wellington enjoying a bye.
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