The 37-year-old former Tottenham Hotspur striker had an MRI on his troubled left knee and is all set to miss the first three matches.
"He has an injury to his left knee and has flown back home. He may be available for their fourth round match against Chennaiyin FC (on December 7)," an ISL official told PTI.
Being their marquee player, Keane was expected to be part of the ISL media event in Kolkata tomorrow but since he has returned home, the ATK will make available some other player.