The 32-year-old, a key member of Japan's last two World Cup sides, hobbled out of Thursday's training with his J-League club Yokohama F-Marinos.
Yokohama officials told Reuters on Friday Nakazawa's participation in the January 7-29 Asian Cup was "unclear" but his absence would be a major blow to Japan coach Alberto Zaccheroni.
"We have been told it will take six weeks to fully heal," Yokohama's Masanori Hakamada said. "But then he has to complete rehab so it's difficult to say if he will be fit in time."
However, the Japan Football Association (JFA) said that Nakazawa could still play a part in the Qatar tournament.
"Six weeks gives him enough time," the JFA's Futoshi Nagamatsu told Reuters. "It's a question of how well he recovers but we're not saying he won't make it in time."
Nakazawa popped up with some crucial set-piece goals to help the Blue Samurai win the 2004 Asian Cup in China.
The former Japan captain had just returned from a torn calf muscle before injuring himself in training.
Nakazawa and fellow centre-back Tulio impressed at this year's World Cup in South Africa where Japan reached the last 16 under Takeshi Okada.
Yokohama are currently in a race for third place in the J-League which would qualify them for next year's Asian Champions League.
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