The Manchester United midfielder has been absent since suffering a knee injury against Braga at Old Trafford last month.
Initially, Ferguson said he expected the Japan international to be back in action within four or five weeks. However, speaking ahead of Tuesday's encounter with Galatasaray in Istanbul, Ferguson said: "His progress has been quite slow actually. He hasn't started any outside work and, looking now, he may be another four weeks away."
It is a major blow for the 23-year-old, who has made an impressive start to his United career following a £12million summer move from Borussia Dortmund.
With two goals in his first eight appearances, Kagawa had offered Ferguson a valuable attacking option.
Now he will probably not be seen again until the Christmas programme at the earliest, which is hardly ideal given the strength of competition to get into the United side on a regular basis.
"Overall, he will probably be looking at 6-7 weeks since the injury," said Ferguson.
"The first job now is to get him back."
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