Radamel Falcao failed to respond to Louis van Gaal's pre-match challenge to prove himself but the giant Belgian came off the bench to secure all three points at Loftus Road
United failed to register a shot on target during last weekend's 1-0 defeat at home to Southampton and Louis van Gaal's team did little to test the league's most porous defence in the opening 45 minutes.
Fellaini was introduced at the interval and, after a remarkable point-blank save from QPR goalkeeper Robert Green denied much-criticised United striker Radamel Falcao, the Belgian netted his third goal of the season with 58 minutes played.
Green's opposite number David de Gea continued his superb individual season by twice thwarting Charlie Austin when the game was goalless and United needed an injury-time strike from another substitute, James Wilson, to make absolutely sure of the points.
|VIEW FROM LOFTUS ROAD
|By Greg Stobart
That was far from convincing from Manchester United but they got the job done and recorded just their third away victory of the season.
Louis Van Gaal’s side were dreadful in the first half but improved in the second once they reverted to a four-man defence against a QPR team that struggled to create chances.
Substitute Marouane Fellaini fired the visitors’ opener into the roof of the net from 12 yards before James Wilson sealed the points late on with a tidy finish.
The youngster showed Radamel Falcao how it’s done after the £240,000-a-week forward failed to respond to the challenge set by Van Gaal this week to prove himself, missing several excellent opportunities to score.
QPR boss Harry Redknapp, who dismissed speculation over his future in the build-up, recalled defender Nedum Onuoha and forward Bobby Zamora in place of benched pair Steven Caulker and Adel Taarabt.
There were defensive recalls for United duo Marcos Rojo and Jonny Evans, while Falcao replaced Robin van Persie (ankle).
At his pre-match press conference, Van Gaal urged Falcao to prove his worth to Manchester United and he passed up a glorious opportunity to do just that in the 13th minute.
An arching left-footed pass from Juan Mata unlocked the QPR defence, but Green stood firm to repel Falcao's close-range effort and then diverted the rebound for a corner.
Eduardo Vargas prodded the ball onto the roof of the net after United dealt unconvincingly with Clint Hill's long throw, and a raucous Loftus Road crowd enjoyed the visiting defence's struggles under pressure during the first half.
In the 19th minute, Zamora capitalised on sloppy play from Evans to play in Austin, but De Gea denied QPR's top scorer.
The Spain goalkeeper bettered that stop to frustrate Austin again with half an hour played, extending himself to turn a sweetly struck 25-yard effort behind.
There was a change apiece from each side at the break as Caulker and Fellaini replaced Richard Dunne and Mata respectively.
QPR midfielder Leroy Fer should have done better when he towered above the United defence to connect with Joey Barton's 51st-minute free-kick, before Green provided a moment to savour.
Angel Di Maria cut back on the right flank to supply a wonderful cross - Falcao seemingly certain to head home, only for Green to push the ball to safety with razor-sharp reaction.
But there was nothing Green could do when United went ahead.
Wayne Rooney released Antonio Valencia on the right flank and Fellaini took a touch before rifling the wing-back's cutback high into the net.
QPR pressed hard for an equaliser - Austin firing wide before Vargas woefully wasted an opening on the counter-attack having initially pounced on a Rojo error.
There was more misery for Falcao inside the final quarter of an hour as he failed to convert Wilson's low cross with the goal gaping, and the youngster beat Green at the second attempt to show his illustrious team-mate how it was done after De Gea had prevented Vargas from earning a share of the spoils.