Blades fan Sharp, signed from United in a £1.1million summer deal after spending last season on loan at Rovers, slammed home a rebound shortly before half-time, nine minutes after James Coppinger had fired the home side into a deserved lead.
Sharp's goal celebrations were muted, but jibes from Blades fans still hurt, so the 24-year-old will have taken immense pleasure in revealing the words 'fat lad from Sheffield' on his undershirt.
Sharp, who failed to live up to his billing during two spells at Bramall Lane, cut a forlorn figure when he limped out of the action just minutes later, but the damage had been done and the visitors never recovered after being given a first-half runaround.
Defenders James O'Connor and Shelton Martis, midfielder Martin Woods and striker James Hayter returned for Doncaster, while Ritchie De Laet, on loan from Manchester United, was back for his last game for the Blades.
Rovers twice threatened to take an early lead, with Woods dragging his shot from the edge of the penalty area just wide after 41 seconds and Coppinger forcing Blades goalkeeper Steve Simonsen into a low save soon after.
John Oster then stepped off his left foot on the edge of the area to create space, but his right-footed shot curled off target.
The Blades had to wait until the 16th minute for their first effort on goal, with Mark Yeates' weak overhead kick comfortably gathered by Rovers goalkeeper Neil Sullivan.
Rovers striker Hayter required lengthy treatment after being caught by Blades skipper Chris Morgan's elbow midway through the first period.
Whether a deliberate act or not, Morgan went unpunished by Hertfordshire referee Keith Hill, but Rovers responded in style to take the lead in the 33rd minute.
Coppinger ran on into the box after laying off a cushioned header to Sharp and, when the return pass fell invitingly into his path, the Rovers winger buried an angled shot beyond Simonsen into the bottom corner.
Rovers doubled their lead nine minutes later with another quality goal.
Oster set full-back Joseph Mills free down the left and his pinpoint cutback was met first time by Hayter. The striker's shot was parried by Simonsen, but only as far as Sharp and the former Blade lashed home the rebound.
Sharp's joy was short-lived though as he limped off due to a hamstring strain on the stroke of half-time and was replaced by Dean Shiels.
The Blades sent on Jamie Ward for Daniel Bogdanovic six minutes into the second half and then Matt Lowton replaced Johnny Ertl, but the visitors still struggled to make headway.
All the cohesion was Doncaster's and that was underlined by a sweeping move involving marauding right-back Mustapha Dumbuya, whose fine ball to the edge of the area was laid off by Hayter for Woods to curl a shot over the crossbar.
The best United had to offer in the closing stages was Yeates' right-footed curler, which was comfortably gathered by Sullivan.