The 24-year-old has yet to play for the Hammers since making his loan move from Liverpool a permanent one because of a heel problem, and injured his foot further during a training session this week.
The plantar fascia is the tissue which supports the arch of the foot and Hammers boss Sam Allardyce is not aware of how long the ex-Newcastle man will be missing for, but does not expect him to require an operation.
"It's the same type of injury but in a different area. It is not an Achilles injury, as has been suggested but instead it involves the band of tissue which runs under the foot," Allardyce told West Ham's website.
"According to the specialist the chance of a recurrence of that type of injury is about four per cent but unfortunately for Andy he was one of the unlucky ones.
"We had been very cautious, very patient with him along the way and it's sad this has happened for him as well as us.
"He doesn't need an operation but we need to give the foot time to re-heal, continue the rehab and get him back in training again."