Japan take on Paraguay on Tuesday with Celtic playmaker Shunsuke Nakamura's fitness in doubt after he returned home with his third Scottish Premier League title -and pain in his left leg.
Brazilian-born centre back Marcus Tulio Tanaka, nursing an injured right shoulder after Japan's 1-0 friendly home win over Cote d'Ivoire on Saturday, may also sit out the hosts' final match in the three-nation Kirin Cup.
Paraguay and Cote d'Ivoire drew 1-1 on Thursday in the tournament, which Japan, the former Asian champions, are using to rebuild their stuttering FIFA World Cup qualifying campaign.
Paraguay, missing key players including Blackburn striker Roque Santa Cruz and Benfica forward Oscar Cardozo, hope to test domestic players before resuming their own FIFA World Cup bid next month at the top of the South American zone table.
Nakamura, whose trusty left boot has inspired many a victory for club and country, hurt his left thigh in training four days before he played in Celtic's title-clinching 1-0 win over Dundee United on Thursday.
"I feel something strange in the back of my left thigh. It's not a kind of fatigue. But it's not so serious," he told reporters after a solitary workout at Japan's training camp here on Sunday when other teammates played a match against a university squad.
The 29-year-old midfielder, due to undergo further medical checks, sounded both cautious and optimistic.
"It doesn't mean anything if my leg is broken," he said. "I played against Dundee United. I think I can play against Paraguay."
But Japan coach Takeshi Okada was more guarded.
"If it recurs, it will lay him off for a long time. I must not push him too much," said Okada, who has called up Europe-based players for the first time since taking over from Ivica Osim in December after the Bosnian suffered a stroke.
Paraguay coach Gerardo Martino has said he wanted to use the Kirin Cup as a "chance to watch as many players as possible."
The Kirin Cup is Japan's first international appearance since they suffered a surprise 1-0 defeat away to unfancied Bahrain two months ago in a FIFA World Cup qualifier.
Bahrain stand top of the Asian Group 2 table with two wins and six points, ahead of Japan and Oman on three points and winless Thailand.
Japan, bidding for their fourth FIFA World Cup finals, will host Oman on 2 June, travelling to Muscat for the return on 7 June, then meet Thailand on 14 June before flying home to play Bahrain on 22 June.