The 31-year-old tested positive for a banned substance at the time of the two legs of the Championship play-off semi-final against Preston North End in May.
The Republic of Ireland international now faces a ban of up to two years which could force Blades manager Kevin Blackwell to sign a replacement.
The only other senior goalkeepers at Bramall Lane are Ian Bennett and Justin Haber.
Middlesbrough are to investigate why several first-team players failed to report back to the club on the first day of pre-season training.
Reports claim that Sanli Tuncay, Mido and Afonso Alves were among a number of players who did not turn up at the Championship club's Rockliffe Park training ground on Monday.
Tuncay is believed to have been given permission to extend his summer break, along with Adam Johnson following his exploits with the England Under-21 side this summer.
Bristol City have completed the signing of Scotland international Paul Hartley on a free transfer following his release from Celtic.
The former Hoops and Hearts talisman has penned a two-year contract at Ashton Gate after his Parkhead deal expired at the end of last month.
Watford have secured the signing of Danny Graham from Carlisle United.
Ipswich Town have completed the signings of Lee Martin and Damien Delaney.