The Greek Under-21 international captain has cost United an initial fee of pounds 300,000, which could rise to pounds 700,000 depending on appearances and performances.
At 6ft 4in, he could prove to be a towering presence for the Blades in their effort to gain promotion from the First Division. The club's caretaker manager, Nigel Spackman, said: "He has a lot to learn about the game but he has a great future in front of him and all the attributes to be a great player and that's why we have taken a chance in signing him.
"He is still young and although I have not seen him live, I have watched plenty of videos and I feel he will be an asset to us and he will fit into the English game."
Dellas speaks little English but will be taking lessons with the pounds 750,000 right-back Borbokis, who has scored in his first two appearances for the Blades.
Chelsea's Frank Sinclair has been charged with misconduct by the Football Association for dropping his shorts in celebration of his goal in the 3-2 defeat at Coventry last Saturday.
Steve Double, the FA spokesman, said: "Frank Sinclair has been charged with misconduct and he now has 14 days to request a personal hearing."
The Rangers striker Brian Laudrup will miss Denmark's Group One World Cup qualifier against Bosnia in Sarajevo on 20 August because he has chicken pox. The same match marks the return as captain of his brother Michael, who suffered a knee injury in April.
"Brian Laudrup has been told by his medical adviser that he will not be able to train again for a further 10 days after his chicken pox, meaning he will have to miss the Bosnia qualifier," the Danish FA said.
Stockport County have signed the Hamilton centre-half Martin McIntosh for pounds 100,000.Reuse content