The 21-year-old, who was called up for Bafana Bafana's COSAFA Cup campaign in the North West last year and for Amajita's U-20 World Cup trip to South Korea, played 17 games for the National First Division side last term.
Mthethwa initially went through Bucs' induction programme for the coming season along with Brian Hlongwa and recruits Brilliant Khuzwayo, Ben Motshwari, Vincent Pule, Meshack Maphangule, Linda Mntambo, Paseka Mako, Abel Mabaso, Kudakwashe Mahachi and Asavela Mbekile.
However, new Richards Bay coach Brandon Truter says Mthethwa is heading back to KwaZulu-Natal.
"The only player that is coming back on loan is the Orlando Pirates youngster Sandile Mthethwa," Truter tells KickOff.com.
"That is not one we have on loan [yet]."
Truter is currently assessing what he has at his disposal for 2018/19.
"A lot of the players were signed on two to three-year contracts so they were very ambitious last season.
"Basically I inherited the squad with the chairman not wanting to release [players] and having to get a new squad.
"So I've been asked to assess now currently and we'll see where it goes from there.
"There are also a few boys on trials that I'm assessing, but there's not much spaces open to sign," adds Truter, who confirmed former Kaizer Chiefs attacking midfielder Philani Cele has been released by his club.