Ben Foster has been put on the stand by list for England after Green ruptured his groin playing for England B on Thursday.
Alec Chamberlain, Watford's coach said:
"The whole season has been a fairy tale for him: joining Manchester United from Stoke last summer, helping Watford to promotion and then being called up by England."
Foster has been on loan to Watford since he joined from Stoke last summer.