The on-loan Portsmouth defender raced on to Shefki Kuqi's pass and rifled the ball into the top of the net.
The goal seemed to deflate United, although they should have equalised when a Michael Tonge free-kick was headed wide before half-time.
As it was, Kuqi killed off the game on the hour when he poached a second from six yards out.
Sheffield United manager Neil Warnock:
"Apart from a world-class goalkeeping display, the best I've seen for many years, we would've got something from the game.
"They're a good side because they beat us 2-0 and they haven't played very well.
"But we've no chance of automatic promotion now, we've got to get in the play-offs."
Sheff Utd: Kenny, Geary (Liddell 67), Harley, Bromby, Cullip, Morgan, Jagielka, Montgomery (Quinn 66), Tonge, Gray, Cadamarteri (Ward 76).
Subs Not Used: Thirlwell, Shaw.
Ipswich: Davis, Wilnis, De Vos, Unsworth, Naylor, Magilton, Miller, Currie (Horlock 51), Westlake, Bent (Bowditch 88), Kuqi.
Subs Not Used: Richards, Karbassiyoon, Price.
Goals: Unsworth 20, Kuqi 61.
Ref: R Beeby (Northamptonshire).