The Socceroo's half-time introduction breathed life into United's stuttering display but another home draw will do little to appease their tiring home faithful.

Nathan Ellington proved there was life at Watford after Marlon King by firing Watford into a 20th-minute lead, but United fought back through Carney midway through the second half.

The Blades showed no signs of the quality that helped upset Manchester City 2-1 in Sunday's FA Cup fourth-round clash and Robson will know that time is running out to find form in the Coca-Cola Championship.

Former United favourite Steve Kabba received a warm reception from the Bramall Lane crowd, but he threatened to cause problems in the early stages, showing his strength to edge past Gary Naysmith on the corner of the box, only to run into Blades skipper Chris Morgan.

But Watford had settled the quicker and they were gifted the advantage midway through the half.

Lloyd Doyley set Tommy Smith free on the right flank in acres of space and when he squared to Ellington on the edge of the box, the frontman had the time to look up and bend a low effort past the outstretched right hand of Paddy Kenny.

And it was Ellington who came closest to scoring again 10 minutes before the break, when Mat Sadler lashed the ball across goal, narrowly eluding the former Wigan man.

Rob Hulse had United's best chance with two minutes of the half remaining, but headed over from Naysmith's cross.

And Kabba almost inflicted further frustration after tidy interplay with Smith, but his angled drive was parried away by Kenny.

Robson changed things around at half-time with the introduction of Carney for Lee Hendrie and United responded by showing a greater sense of urgency. Carney provided the impetus for the Blades best move of the match - striding forward with pace through the centre - before offloading to Shelton whose drilled effort was beaten down well by goalkeeper Richard Lee.

But despite an improved work-rate, United were struggling to penetrate a powerful Hornets back-line. Ugo Ehiogu rose highest to meet Stephen Quinn's 65th-minute corner but his header flew harmlessly over.

But as the visitors tired, United's persistence began to pay off and they earned a breakthrough with 23 minutes to play. Gary Speed's long throw from the left was flicked on by Morgan finding Hulse, who arrowed an effort across goal, where it was bundled over the line by Carney.

Watford were visibly rocked and almost let Hulse in for a second three minutes later, only for Lee to gather his awkward bouncing header. United also had claims for a penalty when Shelton lofted the ball into Jay Demerit's body, but all appeals were waved away.

The home side brought on Michael Tonge for a final throw of the dice as Robson pushed for a much-needed winner.

But despite their pressing, the Blades lacked composure in the final third, summed up by Shelton's stumble in the area, five minutes from time.

Watford boss Aidy Boothroyd will go home the happier of the two managers, with the United crowd once again voicing their dissatisfaction with Robson at the final whistle.