The Daily Mail says the midfielder is facing surgery, which if confirmed will mean his recovery will leave sidelined beyond the end of the season.
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Grealish, who is on-loan from Manchester City, reported to Finch Farm on Monday believing he had suffered a calf strain, but further examination revealed a foot fracture.
As it stands, the surgery option is being discussed and a decision will be made next week.
Asked whether a procedure might be necessary, manager David Moyes said on Friday: "That's part of the discussion to be made with the surgeon.
"Obviously we are not going to get good news, but there are no dates I want to give out at the moment. We are waiting on a couple of things coming back."