Hammers boss Sam Allardyce says the 23-year-old's injury is similar to the one that blighted Ledley King's career.
He said: "It is a concern to us because Jack has not kicked a ball since last season.
"Unfortunately that knee is never going to be 100 per cent so he has to manage it. It is not as drastic as with Paul McGrath or Ledley King but it is similar.
"There are times when he gets a lot of fluid in there and he cannot join in training."