The 'Brave Blossoms' won three matches at the World Cup and provided the upset of the tournament with a shock victory over South Africa.
Japan will also host Asia's first Rugby World Cup in 2019 but the country announced in July the proposed new 80,000 national stadium, the centrepiece of the 2020 Olympic Games, would not be completed in time to host matches.
Yokohama Stadium will host the 2019 final instead while Tokyo Stadium will stage the tournament opener.
Highlighting Japan's growing stature in the game, it was also selected to enter a team, named the Sunwolves, in an expanded southern hemisphere Super Rugby competition, but Tew said on Friday governing body SANZAR still had concerns.
"We're all very excited by what the Japanese achieved at Rugby World Cup but we remain pretty nervous about their readiness for Super Rugby, and that's their next step forward," Tew told local media.
"I wouldn't say it's shaky, but we are concerned about any expansion when you bring new teams into a competition, and it takes a bit of time for them to bed down.
"We had a SANZAR meeting in Sydney this week and a Japanese guy gave a very detailed presentation on things in place.
"A lot of that was encouraging, but they haven't announced a coach yet and haven't got a full player roster, so they are two things they've got to put the foot on the throttle for."
The Sunwolves, who will enter the expanded competition along with a team in Argentina and South Africa's Kings, will also play some games in Singapore in a bid to encourage the spread of rugby throughout Asia.
They are scheduled to play their opening match against South African side the Lions in Tokyo on Feb. 27.
While there were concerns about Japan's entry into the competition, Tew said the Argentine team would be a handful for the established sides.
"Argentina will be in very, very good shape," he added.
"They're very well organised, very prepared and they've got a very strong player roster.
"They might actually be a quite considerable worry and they're going to add considerable colour to the competition."
Argentina reached the semi-finals of the World Cup last month, where they lost to Australia.
(Writing by Greg Stutchbury; Editing by Peter Rutehrford)
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