The 23-year-old has picked up a bug which leaves him unavailable for the sold-out encounter at Villa Park.
Villa had won all seven games since Grealish returned from a three-month injury lay-off at the start of March, a run which has seen them surge from mid-table into the fifth place in the Championship.
But they will have to take on their play-off rival Robins without their talisman.
Lee Johnson’s visitors sit one spot behind Villa in the table, having played one game less.
Villa are also without the suspended Tyrone Mings, while Kortney Hause is expected to miss the game through injury.