Japan's quest to retain the Asian Cup title they won four years ago has been severely undermined by a match-fixing scandal engulfing coach Javier Aguirre.

The storm clouds swirling around the 56-year-old Mexican, who will appear in court in Valencia this year following an investigation into a match played in 2011 when he was manager of Spanish club Zaragoza, has caused major embarrassment to the Japan Football Association (JFA) and threatens to unsettle the Blue Samurai at this month's tournament in Australia.

Aguirre has vehemently denied any wrongdoing, but JFA officials are furious about the disruption caused to Japan's Asian Cup defence and potentially the team's 2018 World Cup qualifiers, which begin in June.

Japan, who beat Australia 1-0 in the 2011 final to capture the Asian Cup for a record fourth time, should still emerge from Group D, where they face Jordan, Palestine and 2007 champions Iraq. But Aguirre's summons by Spanish prosecutors has complicated their task.

Knives were already out for Aguirre, who led Mexico to the last 16 of the 2002 and 2010 World Cups, when an alarming start in the job brought an abrupt end to his honeymoon period after taking over from Italian Alberto Zaccheroni following Japan's World Cup flop last year.

But a 6-0 thrashing of Honduras and, more significantly, a 2-1 home victory over fierce rivals Australia in November gave Aguirre -- whose incessant tinkering has done little to inspire confidence in his side -- a welcome boost ahead of the Asian Cup.

Forced to bring back Japan's caps record-holder Yasuhito Endo as a short-term fix after a run of inept performances, Aguirre also saw talisman Keisuke Honda return to his best, although playmaker Shinji Kagawa has yet to rediscover the form that once made him one of the most sought-after commodities in Europe.

Japan have looked defensively frail under Aguirre, allowing Neymar to run riot and score all four goals as they lost 4-0 to Brazil last October. But they have slowly improved in attack, suggesting they will be contenders in Australia.

Japan begin their campaign against Palestine in Newcastle on January 12, while Aguirre is expected to give evidence in court in February.

If found guilty, Aguirre could be sent to jail for up to four years, local media have reported.

Already on thin ice with the JFA, failure at the Asian Cup could trigger an early release from Aguirre's contract, whether found guilty of match-fixing or not.