Amosa was injured in a high tackle while scoring a try in Samoa's 34-9 victory over Russia.
Manu Samoa coach Steve Jackson has expressed his sympathy for the hard-running number eight.
Jackson said he has been an outstanding member of the team and he will be missed.
"We are absolutely gutted for him. He is upbeat - but we just need to do what's best for Afa."
Amosa injured his right knee in the act of scoring, just after halftime, in Manu Samoa's 34-9 victory over Russia on Tuesday night.
Assistant Coach Tuifa'asisina Alastair Rogers echoed Vaeluagaomatagi's sentiments.
"We are just so disappointed for Afa - because he put a lot of effort into the Rugby World Cup. He put a lot of work into his performances and he was playing really well. "
The coaching group is considering its options on who will be called up to join the Manu Samoa Rugby World Cup 2019 campaign.
Their second game is against Scotland on Monday September 30 at 7.15pm Japanese Time.
That means the local timing of the match is:
Apia: Tuesday, October 1 at 12.15am
Pago Pago: Sunday, September 29 at 11.15pm
Auckland: Monday, September 30 11.15pm
Sydney: Monday 8.15pm
Samoa will play its third pool match against hosts Japan on Saturday October 5 at 7.30pm.
Apia: Sunday, October 6, 12.30am
Pago Pago: Friday, October 4, 11.30pm
Auckland: Saturday October 5 11.30pm
Sydney: Saturday October 5, 8.30pm
A week later it will play Ireland on Saturday October 12 at 7.45pm.
Apia: Sunday October 13 12.45am
Pago Pago: Friday October 11 at 11.45 pm
Auckland: Saturday October 12 11.45pm
Sydney: Saturday 9.45pm
When will the final of the Rugby World Cup be held?
The final of the 2019 Rugby World Cup will be held in Yokohama on Saturday, November 2 at 6pm.
The kick-off in local times will be:
Apia: Saturday November 2, 11pm
Pago Pago: Friday November 1, 10pm
Auckland: Saturday 10pm
Sydney: Saturday 8pm