United recorded a sixth successive victory in Saturday's re-arranged league encounter.
First half goals from defenders Ethan Ebanks-Landell and Jack O'Connell and second half strikes from Caolan Lavery and Kieron Freeman, his third in three matches, ended Southend's unbeaten run.
Chief Wilder made two changes but new boy Carruthers had to be content with a position amongst the substitutes. Coutts and Duffy returned to the starting XI with Wright and Scougall dropping down to the bench.
The Blades signalled intent inside 20 seconds; a quick Freeman throw presented a chance but an under-pressure Sharp was off target. The opening goal, however, was just moments away.
Demetriou conceded a corner after making a mistake and touches from Coutts, Fleck and Duffy resulted in a cross from the latter and EBANKS-LANDELL added a deft flick to divert the ball home. Further Blades chances followed but Duffy shot over after a patient build-up and Freeman poked wide.
Despite a lack of early chances, the Southend front three of Cox, Fortune and Ranger saw plenty of the ball and two of the aforementioned players combined to level the scores on 19 minutes. Fortune pulled the ball back from the right and COX slotted through legs and into the bottom corner of Moore's net.
The home side followed up with a spell of pressure, winning a number of corners and Moore had to tip over Wordworth's half-volley after a long throw was only partially cleared. A long-range Atkinson shot also forced Moore into a save on 35 minutes.
United withstood the onslaught and close to half-time the action was very even, until the Blades went back in front on 42 minutes. Ferdinand was penalised for a foul on Lavery and Fleck's free-kick was turned in by O'CONNELL, who snuck in unnoticed at the back post.
The early moments of the second half were even. Freeman's touch prevented Thompson getting a header in and Moore punched away under pressure. United lost Lafferty before the hour, his left wing-back position taken by Done.
Lavery had a shot deflected and on 66 minutes Duffy's final contribution was to cut in from the right and curl over the crossbar. Carruthers was introduced for his debut with over 20 minutes to go.
Carruthers was soon involved, he was part of the build-up as United added a third goal on 72 minutes. Freeman held off a challenge in the box, stayed on his feet and squared for LAVERY, who netted at the second time of asking. The compliment was repaid moments later when Lavery clipped over the defence and FREEMAN got ahead his marker and lifted over Smith into the roof of the net, to the delight of the 1,450 Blades fans at that end of the ground.
Sharp attempted to get in on the action but his half-volley, whilst falling back, was just over and a negative of the second half was Carruthers hobbling off with eight minutes to go.
Southend pulled a goal back late on. Freeman and Ebanks-Landell both had chances to clear and McGLASHAN even slipped over before squeezing the ball through Moore from an acute angle.
In injury-time Cox powered an effort against the Blades crossbar and the home side, expectedly, saw most of the ball, but the rearguard held firm to record another welcome victory.
Southend: Smith, Demetriou, Thompson, Ferdinand, Timlin, Atkinson (Hines 72), Leonard, Wordsworth, Fortune (McGlashan 60), Cox, Ranger
Unused: White, O'Neill, Bridge, Kyprianou, Bexon.
Blades: Moore, Fleck, O'Connell, Basham, Lavery, Sharp, Coutts, Freeman, Ebanks-Landell, Duffy (Carruthers 68, Scougall 82), Lafferty (Done 56).
Unused: McNulty, Ramsdale, Wright, Clarke.