For the second match in succession a second-half in-swinging free-kick helped Manchester United convert an average performance into three points. This time it was Robin van Persie who volleyed in at the far post from Wayne Rooney’s delivery to add to his first-half opener and cruelly render Graziano Pelle’s equalizer for Southampton meaningless.

In truth, average would be a kind way to describe an error-strewn, disjointed display by Louis van Gaal’s men. Southampton were superior, yet, even more so than against Arsenal last week, can count themselves unfortunate to finish empty handed. The much-discussed run of fixtures to test Southampton’s true top-four credentials has finished with Ronald Koeman’s side pointless and now outside of the top four.

In contrast, Manchester United sit firmly inside those Champions League places on the back of five straight victories. They now reside in third spot, the position that the club targeted at the start of the campaign. Nestled eight points behind Chelsea and five behind rivals Manchester City, there may be a temptation, too, to ponder a championship challenge. Yet that would be hugely premature.

United are displaying the classic championship-winning art, one they perfected under Sir Alex Ferguson, of winning when not playing well. And while there is clear credit due to Van Gaal for reintroducing a steely determination, the performances will soon have to catch up with the results, or the reverse will be inevitable.

On the balance of play, they were second best at St Mary’s. Yet United prospered thanks to an early errant back pass from Jose Fonte that allowed Van Persie to sneak in and score, and then none-existent marking and a goalkeeper rooted to his line that presented the Dutch striker a chance to double his tally 19 minutes from time. Van Persie, so lackluster in recent times, produced perhaps his best display of the season. And Untied did get better as the match wore on, eventually seeing the win out without any real cause for concern.

Yet the first-half performance in particular did not make pretty viewing. Playing with a back three, just as they did in claiming their first away win under Van Gaal at Arsenal recently, United struggled desperately to find any fluidity. The high pressing of a Southampton side that, like their opponents, were missing several players due to injury, meant that United were effectively forced into a back five.

It was against the run of play, then, that United went in front. A meaningful chance had yet to be created when, in the 12th minute, Fonte tried to find his goalkeeper without checking for danger and Van Persie showed fine anticipation and hunger to get there before Fraser Forster and finish through the legs of the England goalkeeper.

The goal did little to settle the visitors, however. United’s shaky back line was further unsettled when the man most responsible for their improved defense of late, Chris Smalling, became the latest defender to go down with an injury, replaced by the only just fit again Jonny Evans.

It was only down to the hosts failure to click in the final third, that it took until the 31st minute for Southampton to get on the score sheet. For the second match running, Marouane Fellaini costly gave the ball away in midfield. The midfielder, displaying the form of his dismal first season at Old Trafford rather than his recent rejuvenation, erred again in missing a header from Shane Long’s cross and, after an initial defensive block, Pelle ended a run of five games without a goal with a fierce left-footed finish.

Southampton will rue that they were unable to capitalize further. Long was the primary culprit, blasting over at the near post after a terrible error from young center-back Paddy McNair. It was a sign of United’s problems and especially uncertainty at the back that Van Gaal took McNair off after 39 minutes. With Ander Herrera coming on, it was assumed that a shift in formation was in order. Instead, to the surprise, not least, of the United midfielder, Carrick was moved back into the three-man defense.

More poor defending allowed Southampton further chances after the interval. The best of them saw Long head straight at De Gea with a free header at the back post, while Pelle shot over following a giveaway from Marcos Rojo. Largely dominant in the middle, Southampton were undone by their failings at both ends.

Carrick’s switch allowed United to improve their ability to pass out of the back line and get Ashley Young and Antonio Valencia further up the pitch. But with the visitors failing to create a single chance of note from open play, Southampton will be mystified at how they failed to take gain anything from the contest. And Koeman will doubtless be hugely frustrated, too, at the marking for the decisive goal. While Juan Mata’s free-kick against Stoke City last week went in without the need for a touch, this time Van Persie was left all alone around the back to allow him to cleverly beat a flat-footed Forster with a clinical volley.

De Gea was put into action with 13 minutes left on the clock and made a fine one-handed save to deny Pelle, but Southampton’s imprecision in the final third and the doggedness of United meant there was to be no late drama.