Petkovic and Faty will sit out the round 17 visit of Victory to Allianz Stadium after both players were sent off during Sydney's 3-1 win over A-League leaders Perth Glory last week.
The dismissals in Perth dampened an otherwise successful night for Sydney, who consolidated fifth spot and moved within eight points of Glory.
Sebastian Ryall and Matthew Jurman are set to replace Petkovic and Faty at the heart of Sydney's defence.
"Yes, we lose quality stoppers but it's a big chance for two players with a lot of A-League experience, and both of whom have won a couple of championships," Arnold said following Saturday's win.
"We'll need more of the same against Victory but we also need a big crowd to help us improve our home form.
"Away, we're unbeaten, and if our home form picks up then anything is possible. We've got two home games coming up and most of all I'm looking for consistency."
Leading into Saturday's fixture, coach Ernie Merrick revealed he is in talks with the club regarding a new deal.
"I've only just started having those discussions but I am enjoying Wellington and want to stay," Merrick said."I think, and hope, the board feels the same way."
The match is vital to City's finals chances, after their derby loss to Victory left them just a solitary point clear of in-form defending champions Brisbane Roar.
Saturday's other fixture sees Newcastle Jets and the Wanderers go head-to-head at Hunter Stadium.
