Casillas appears to be staying at the Bernabéu with Madrid, with Arsene Wenger expected to go back in for Marshall as he looks to provide stiff competition for current number one Wojciech Szczesny.
Arsenal reportedly had a £6million bid for Marshall rejected next week, and are now expected to up their offer to £8million for the Bluebirds star.
Other reports had suggested that Arsenal could pay up to £15million for Marshall, a price that would put him in the top-five most expensive goalkeepers of all time. But manager Arsene Wenger appears to be biding his time in order to get a better deal, offering just over half that reported price.
Marshall was one of the best goalkeepers in the Premier League last season, and topped the saves chart with 153 stops during an otherwise miserable campaign for the Welsh club.
The Scottish international has done little to suggest he is desperate for a move to a Premier League club, even stating that he would happily help try to get Cardiff back to the top-flight next season.
Another rejected bid from Cardiff would likely test Arsenal's patience in their pursuit of Marshall, and could see them search elsewhere.
Former Barcelona keeper Victor Valdes is a free agent and has been on Ligue 1 outfit Monaco’s radar, but that is a move that is now considered unlikely to transpire.
For now Cardiff must wait for another official bid from Arsenal, but if an £8million evaluation is deemed unacceptable by the Bluebirds, it could be the last chance they get to sell their Player of the Year.