A procession of chances came and went in-between the two strikes, meaning much of the evening made uncomfortable viewing even though United - and in particular the front pairing of Rooney and Ryan Giggs - often had Portsmouth chasing shadows.
After Sunday's FA Cup semi-final marathon against Everton, Sir Alex Ferguson made wholesale changes for the second successive game, with a string of big-name players returning. Two of them - Giggs and Rooney - combined sumptuously to put United ahead inside nine minutes.
Darren Fletcher, back from injury and on the lung-busting form of recent weeks, won the ball and it was quickly ferried forward to Anderson, whose excellent pass allowed Giggs to spring the visitors' offside trap. Waiting in the centre was Rooney, who finished with aplomb after a perfect rolled cross from his veteran colleague.
Amid the joy and release of any early tension, attentions switched to skipper Gary Neville, who sat stricken in his own half after sustaining a foot injury. After consultation with the club's medical staff, the club captain was promptly replaced by John O'Shea.
With the prospect of a three-point lead looming, United were clearly intent on also boosting a goal difference which had been overtaken by second-placed Liverpool in recent weeks, and the chances came thick and fast.
It was O'Shea who came closest to doubling the Reds' advantage before the break, as he headed Giggs' left-wing corner past James, only for Nadir Belhadj to smuggle the ball off the line and away to safety.
Next, a mis-timed header from Sol Campbell allowed Ronaldo to break down the right and cross low across the area for Giggs. His delicate chip beat James, but also drifted just past the far post. The 35-year-old held his head in his hands, aware that a glorious chance had just gone begging.
It looked like Giggs had made amends a minute later by feeding Rooney to round James and score, but a linesman's flag belatedly - but correctly - chalked it off. Quite how half-time came and went with United only one goal ahead was a mystery.
The fact that so many chances had come and gone seemed to increasingly play on United's minds as the second half unfolded. The Reds' cause wasn't helped when O'Shea suffered an injury after a clash with Belhadj, who appeared to stand heavily on the Irishman's foot. With the injury count rising but the goals tally stuck on one, a nervous atmosphere began to brew within Old Trafford.
The tension was never higher than around the 70th minute, when Pompey mustered two particularly threatening forays forward. Peter Crouch, who ploughed a lone furrow without much joy but with plenty of effort, first volleyed wide, then saw his header well saved by Edwin van der Sar.
Rooney saw another shot well saved by James after latching onto van der Sar's long punt, but a priceless second goal did finally arrive with eight minutes left. Carrick, thrown on at the expense of Anderson, came up with the goods and Scholes, fittingly on his 600th United appearance, was the architecht.
Just as he has done umpteen times in his 15-year senior career, the midfield magician exploited a gap before the Pompey defence were able to plug it by slipping an inch-perfect pass into the run of Carrick, who finished clinically into James' bottom-right hand corner.
With the three points assured, United began to press forward with more abandon, striving to make up further goal difference. Ronaldo had a goal chalked off and also fired over while Rafael was denied by James, leaving the Reds still three behind Rafael Benitez's men on that count.
More importantly, however, the telling gap between champions and chasing pack is now three points, with a game in hand still to make up.