Michael Carrick became the second former Hammer to score, just before the hour mark, to put the result beyond doubt, meaning that a victory over Wigan next weekend will make United league champions for the 17th time.
Despite their exertions on Tuesday evening, Sir Alex Ferguson named the same starting eleven which overcame Barcelona, while Anderson, Ryan Giggs, Darren Fletcher, John O’Shea and Tomasz Kuszczak remained on the bench.
That meant no return for Nemanja Vidic, still sidelined on doctor’s advice after suffering a concussion against Chelsea, and Wayne Rooney, who continued his recovery from a hip injury.
Despite the absence of their main striker, the Reds were ahead after just three minutes as Ronaldo ended a goal drought – lengthy by his standards – of four games.
Picking the ball up on the right flank, the winger bamboozled Hammers skipper Lucas Neill, advanced into the area and fired a low shot inside Rob Green’s near post, via a slight deflection off George McCartney.
The perfect nerve settler for United in a must-win game, although there was a collective bout of breath-holding at the Stretford End three minutes later, as Rio Ferdinand and Patrice Evra combined to clear Bobby Zamora’s header off the line.
The visitors’ deployment of two strikers, allied to United’s need for victory, made for an open, entertaining start to the game. While clear chances were at a premium, both sides were going hell for leather in search of them.
Moments after Wes Brown had survived an appeal for handball inside his own area, former West Ham striker Carlos Tevez saw a wicked cross cleared by James Tomkins, with Ronaldo lurking at the back post for a certain tap-in.
Unfortunately for the young Hammers defender, he was unable to prevail in a repeat scenario after 24 minutes. Hargreaves manoeuvred space on the right wing and curled a left-footed cross towards the back post, Tomkins misjudged the ball’s flight and Ronaldo was on hand to turn the ball over the line from six yards.
There was no blame to be attached for the Reds’ third goal, which came two minutes later. Tevez received the ball on the left flank, motored inside and thumped an unstoppable 30-yard effort over the despairing dive of Green.
It was a stunning goal from the Argentinean striker, but his refusal to celebrate against his former side drew warm accolades from both sets of fans.
A thrilling game continued apace, and West Ham quickly established a foothold with a brilliant goal of their own. Brown couldn’t fully meet a chipped pass from Hayden Mullins and, as the ball looped up, Dean Ashton contorted brilliantly in mid-air to lash an overhead kick past the motionless Edwin van der Sar.
With the game less than half an hour old and four goals already chalked up, this was a return to the swashbuckling entertainment to which Old Trafford is accustomed, following Tuesday’s cagey, nerve-fraught victory over Barcelona.
With West Ham reducing the deficit, there was still room for nerves, however – and they were heightened further when United were reduced to 10 men shortly before half-time.
Nani tangled with Neill off the ball, the Hammers skipper pushed the young winger, who responded by nudging the Australian with his head. Both players fell theatrically to the ground and, after seeking the advice of his assistant, referee Mike Riley brandished the red card to Nani.
Sir Alex Ferguson responded by sending Ronaldo to his traditional role on the wing, leaving Tevez as the lone striker in a 4-4-1 formation. The goal plundering would have to wait, preserving the advantage was now the name of the game.
Taking advantage of acres of space in the centre of the park, Carrick strode into space and hit a low shot goalwards. Sweetly for United, the effort again took a deflection and bypassed the helpless Green. As an added bonus for the home contingent, the telling nick came off Neill.
Nerves now banished, United retained the attacking mantle. Scholes almost added to his midweek strike against Barcelona, but was thwarted by a last-ditch challenge from Parker, while Darren Fletcher sidefooted against the post after lovely approach play from fellow substitute Ryan Giggs.
West Ham’s attacking ambitions were almost extinguished, save a fierce pot-shot from striker Bobby Zamora which van der Sar did well to hold, and United were able to see out the remainder of the match in relative comfort.
Second-placed Chelsea travel to Newcastle on Monday, looking to make up United's three point lead. Whether they do or not, the Reds know that victory over Wigan at the JJB Stadium next weekend will be enough to retain the Premier League title.