Fainga’anuku played seven tests in 2022 and 2023 and was a breakout star at the last Rugby World Cup, scoring five tries.
Tavatavanawai will miss the remainder of the Rugby Championship after breaking his arm in Tasman’s NPC clash with Otago on Saturday.
The midfielder fractured his radius and faces 12 weeks on the sidelines.
New Zealand Rugby released an update of player injuries ahead of Thursday’s team naming for the Eden Park test against the Springboks.
The biggest issue remains at halfback, with Cam Roigard and Noah Hotham still facing up to four weeks before returning to action while Cortez Ratima is in doubt for Saturday’s test, with Finlay Christie and Kyle Preston remaining with the squad.
Lock Patrick Tuipulotu is also three to four weeks away, meaning a potential return in the Bledisloe Cup.
On the positive front, prop Tyrel Lomax is back fit after a thumb injury, while midfielder Anton Lienert-Brown has also been cleared after a concussion in the opener against Argentina.
Wing Caleb Clarke and loosie Luke Jacobson will get another NPC runaround before they’re considered for test selection.