Carsley, 33 has done the rounds in the top flight since the Rams were promoted to the Premier League in 1996 and has stayed in the Premiership with Blackburn Rovers, Coventry City and present club Everton.
He has over 300 Football League appearances to his name as well as 29 Republic of Ireland caps.
Blades boss Bryan Robson would initially plan to being in the midfielder on loan with the anchorman valued at £1,000,000 by the Toffees.