The 31 year old full-back has joined from Pompey on a three year deal at Bramall Lane. The Blades will on pay a fee if the club are promoted to the FA Premier League.
''I'm dropping out of the Premiership but just for a season.'' Unsworth told BBC Radio Sheffield.
''This club can get promotion with the squad that is here, I am certain of that. I have been impressed with what I have seen here.''