Coach Steve Jackson has confirmed this after Amosa sustained a tournament-ending knee injury during Samoa’s 34-9 victory over Russia on Tuesday.
"Alamanda was part of our group for the Tonga test in the Pacific Nations Cup, so he understands our game and our team environment. Even though he is a genuine number seven – he is also very versatile and can cover multiple positions. He’s a very fit and hardworking player who deserves the opportunity," Jackson said.
Motuga played for the Manurewa club in New Zealand.
The 25-year-old has also been a stand out player for Samoa on the World Rugby International Sevens Series, where he has scored 55 tries from 161 matches.
His single Test appearance for the Manu Samoa was against Tonga during the Pacific Nations Cup in July.
Motuga arrives in Japan on Friday.