Mphahlele feels there is still plenty more to come from the Amakhosi strike-duo, who both found the back of the net against SuperSport United over the weekend.
Billiat opened the scoring before Castro later found an equaliser to ensure Chiefs left Lucas Moripe Stadium with a healthy away-goal advantage, after the two sides played to a 2-2 draw in their MTN8 semi-final first leg on Sunday.
The former Mamelodi Sundowns pair, who famously formed part of the 'CBD' trio which included Keagan Dolly, have shown signs of re-establishing their fruitful partnership under Giovanni Solinas this season.
Mphahlele believes scoring is guarenteed with Billiat and Castro on the pitch, but has since called on the defence to sharpen up in order to contribute with clean sheets and ensure victory for the team going forward.
"[Scoring] gives us big confidence, these guys [Castro and Billiat] are top players. You're guaranteed that you will get goals when they are on the pitch," Mphahlele told MTN8 TV after Sunday's clash.
"We have to work together with them and try to help them, especially us as the defence. We have to try and keep the zero [in the scoreline], and we know they will score.
"We know Khama is going to get something; Castro is going to get something, but it's a work in process. We have a new man in charge, we have to keep working and fix it, but the journey [to the MTN8 final] continues."
The Glamour Boys will momentarily turn their focus back to the league to face Bloemfontein Celtic on Wednesday evening, before they welcome Matsatsantsa to the FNB Stadium on Saturday for the semi-final decider.