But Watford, whose cup hopes ended in a 4-1 defeat by Wolves, held firm to hang onto a point after David Carney had cancelled out Nathan Ellington's first-half strike
The Vicarage Road side, who have stuttered in recent games after a flying start to the season, almost found a way through early on.
John-Joe O'Toole picked out Jobi McAnuff and the midfielder pumped the ball to Ellington, who miscued six yards from goal.
Lee Hendrie tested Richard Lee with a low strike from just inside the area, before Watford snatched the lead in the 21st minute.
Former United striker Steven Kabba threaded the ball to Lee Williamson and he slipped it inside to Ellington, who had time to pick his spot off the inside of the far post for only his second goal since his move from West Brom last summer.
United were almost caught out on the break 15 minutes later when a long ball upfield found McAnuff on the left flank.
The midfielder pumped over a low cross which fell to Ellington, but he spun quickly before drilling in fierce shot which was deflected wide of the target.
The Sheffield side, struggling to find their rhythm, were almost level when full-back Gary Naysmith picked out Rob Hulse with his precise cross from the left flank, but from six yards the striker powered his header over the target.
Paddy Kenny foiled Watford minutes later by blocking Kabba's angled drive at the near post with his feet.
United's reply was immediate as they broke quickly and Lee Martin crossed for Gary Speed whose header was snatched on the line by Lee.
Watford survived in the 56th minute when Lloyd Doyley conceded a free-kick which ended with Luton Shelton hitting a tame shot straight at Lee.
But United were back on level terms in the 67th minute. Speed's throw was flicked on by centre-back Chris Morgan to Hulse, whose short pass was finished off by Carney.