Sundowns were humbled 3-0 by the Students last weekend, dampening their spirits ahead of the first leg of their CAF Champions League final with Zamalek in Atteridgeville on October 15.
Castro, Billiat and Keagan Dolly shone upfront in the Brazilians' league and cup double last season as 'CBD' - a nickname inspired by Barcelona's 'MSN' trident of Lionel Messi, Luis Suarez and Neymar.
Asked if Castro's unavailability through injury was felt against Wits, Billiat said: "I think Castro is a good player, but that shouldn't be an excuse. We have to do our job as professionals.
"It's a wake-up call for us. It's things like these that can make us a better team," Billiat added on the sidelines of yesterday's Telkom Knockout draw, which pitted holders Downs against Polokwane City.