Mathoho moved to Kaizer Chiefs recently and Celtic have signed two former Chiefs defenders; Dominic Isaacs and Brent Lawler, who were fringe players in Naturena.
Nthethe says the duo is capable of filling the void left by the Venda-born player.
"What I can say is Mathoho is a good player and it was good to play with him, but I can tell you that we didn't lose that much because we still have players like Dominic Isaacs and Brent Lawler… They can cover that space and I don't think that will be a problem," Nthethe says.
"I don't think it is a big loss, although we wanted to keep him but we managed to get good players to replace him," he adds.
Siwelele's defence will be tested on Friday by Orlando Pirates when the two sides meet in the MTN8 quarterfinals clash on Friday at the Orlando stadium, but Nthethe is adamant Celtic are up for the fight.
"We know it will be a tough game, but then again we are ready to play any team… I think we have an exciting team for this season. I can promise our fans that we won't disappoint them this season," Nthethe concludes.