Forster fired the winner shortly before the interval after Ashley Ward had cancelled out Adrian Williams' early goal to keep Reading handily placed for their own assault on the promotion places.
Reading boss Steve Coppell handed a debut in defence to Ivar Ingimarsson, yesterday's £100,000 capture from Wolves, while Jack Lester earned a start for United following his goalscoring cameo as a substitute in Tuesday's 1-1 draw with Wigan.
And it was the visitors who made the early breakthrough when skipper Williams lost his marker and met Nicky Shorey's sixth-minute corner with a firm near-post header. United 'keeper Paul Gerrard got a hand to the ball but could only help it into the back of the net.
The Blades hit back barely three minutes later when Stuart McCall caught the Reading defence napping.
The veteran midfielder's quickly-taken free-kick sent Michael Brown scampering down the left, and his cross was met by the onrushing Ward who converted from close range.
Visibly buoyed by the equaliser and with Reading being out-muscled and out-thought in midfield, United dominated possession and Royals 'keeper Marcus Hahnemann did well to keep out a Phil Jagielka header.
Mike Whitlow then slung in an inviting cross which Brown headed narrowly wide before the move of the half culminated in Hahnemann producing a fine reflex save from Ward.
Hahnemann then acrobatically denied Robert Kozluk as United turned the screw, but Forster had other ideas and silenced the home crowd by striding in unchallenged from the left before firing an unstoppable shot past the despairing dive of Gerrard.
For all United's industry and endeavour it was Reading who were creating the better chances early in the second half.
Williams should have done better with a shot from just outside the box before John Salako and Scott Murray both warmed Gerrard's gloves from long range.
And when Lester did manage to wriggle through a gap in the Reading rearguard he was thwarted by a fine last-ditch block from Williams.
Lester's night came to a premature end when he was replaced by Paul Peschisolido, but he struggled to get involved as time and time again United's final ball let them down.
The former Birmingham striker thought he was through on goal late on but Ricky Newman got a vital foot in to ensure Reading hopped up to eighth in the table while the Blades drop to fourth.