Hamilton and Anderson will also miss the following games against Peterborough and Hartlepool.
The trip to Ashton Gate will be on of the Mariners toughest games of the season, one where Paul Groves could have done without missing three of his key players.
John McDermott or Jason Crowe are expected to replace Cas at right back, whilst Groves will have to pick between himself and Chris Bolder to compensate the loss of Hamilton in midfield.
It is Anderson's left wing slot that will cause the problems though with Stuart Campbell needed in the centre. Youngster Graham Hockless would seem to be the favourite to occupy the berth after an impressive pre-season.