Failure to take their chances could well prove costly when the play-off places are decided but United were not the only culprits.
Brighton also missed their share and were caught out by the offside flag on three occasions, including Glenn Murray's disallowed goal.
The pressure of trying to qualify for the play-offs was evident in a match containing too many fouls and mistakes in the difficult conditions.
Dougie Freedman was denied by an excellent goal-line clearance from Andy Whing in the 14th minute and then rolled a shot inches wide.
The impressive Jonny Howson was the victim of a full-stretch save by Brighton keeper Michel Kuipers as United dominated the first half.
Although Brighton saw more of the play after the break, United substitute Tresor Kandol wasted an excellent chance in injury time when Jermaine Beckford picked him out.
Victory is now vital at third-placed Doncaster as left back Alan Sheehan recognises. He said: "Tuesday night's game is a must-win. We need the three points off our promotion rivals but at least we kept another clean sheet against Brighton."
Sheehan, on loan from Leicester, is coming on in leaps and bounds after receiving coaching from Steve Staunton and his forward runs on Saturday were a feature of the game.
Gary McAllister said: "We huffed and puffed and did our best but it was two points dropped. We need to reverse that by winning at Doncaster. They are all massive games now, whether they are against teams at the bottom fighting for their lives or like us, going for promotion.
"Brighton came for a point and got one. They played the same way at Nottingham Forest last Monday night so we knew what to expect, but we missed too many chances."
Bradley Johnson is doubtful for the derby after taking several knocks so Peter Sweeney or Seb Carole could cover.