Manchester United have not been shy about spending their way out of trouble this season – and, in turn, scoring their way back into the title race. It did not take long for new boss Louis van Gaal to decide that, in addition to the summer swoops for Luke Shaw and Ander Herrera, more attacking options were required. Cue a British transfer record deal (£59.7m) for Angel di Maria and an opportunistic loan deal agreement that brought Radamel Falcao to Old Trafford.

And the club's fortunes are starting to change, with back-to-back wins over West Ham and Everton. There are still issues to address, but the league table currently makes much happier reading for Manchester United supporters.

Without doubt, the defence remains the major Achilles heel, even after Van Gaal moved away from his preferred 3-5-2 formation, but there is even encouraging news there, with 19-year-old Paddy McNair emerging as a future star. McNair impressed initially with a resolute performance against West Ham and has not been daunted by his unexpected call-up in the wake of an ongoing injury crisis among Manchester United’s recognised defenders.

But McNair's chance to step up has cast light on a perplexing question that has been somewhat overshadowed in the rush to assess Manchester United's potent attack. Not so long ago, Phil Jones and Chris Smalling were considered as the central defensive partnership of the future for Manchester United and for England. But a string of untimely injuries, coupled with some disappointing performances, have derailed both players' progress, leaving Van Gaal short of options as he looks to replace Nemanja Vidic and Rio Ferdinand, who both called time on their Old Trafford careers this summer.

Jones arrived with plenty of fanfare back in 2011 after catching the eye at Blackburn. While centre-back has long been viewed as his best position, he has logged the majority of his career minutes prior to this season either at right-back or as a defensive midfielder. A groin injury has been the latest setback, but the 22-year-old has already battled knee and back problems in his short career to date.

Smalling, meanwhile, has fought a host of injuries of his own since joining Manchester United from Fulham – a groin injury and a broken metatarsal among them as well as the current thigh issue. Like Jones, he has played out of position at right-back on numerous occasions but, with a central defensive spot now his for the taking, the 24-year-old just cannot stay fit.

The fact that Jonny Evans has also been in and out of the treatment room has made it even tougher to move on from the Vidic-Ferdinand era. Evans, now 26, was thought to be Van Gaal’s top choice at centre-back heading into this season, but the Northern Ireland international has had some nightmare afternoons. He is out for a couple more weeks after picking up an ankle injury in the miserable 5-3 capitulation against Leicester.

In truth, the injuries have made it impossible to fully evaluate whether Manchester United have the long-term defensive solutions in-house or need to scour the transfer market. But, until Smalling and Jones can stay fit for a full league campaign, it is hard to think of that pairing as a cornerstone of the Red Devils' future.

For now, the positions at the heart of the defence belong to McNair and Marcos Rojo but Van Gaal will have tricky choices to make when his trio of injured centre-backs return to action. Manchester United have some huge Premier League matches ahead, including a massive clash with leaders Chelsea, and they desperately need continuity at that position as they try to shrug off a slow start.

The Red Devils boss tried to shift the spotlight away from his defence with recent comments, but that is easier said than done given the fragility on show each week. “I want my defensive organisation to be better but the defence begins in the striker’s position”, he said. “We have a lot of creative attacking players in our selection and I like that but I also like to think that you have the discipline to come back in a certain shape because now we have conceded too many goals. The media are writing it’s because of the weak defence, but no.”

One thing is clear: despite all the attacking stars at Van Gaal’s disposal, the Red Devils cannot force their way into the title race without a reliable defence just ask Kevin Keegan how an all-out attack approach worked for his Newcastle side. Depending on what the Dutchman sees from his defenders over the next three months, he may have to venture back into the transfer market in January. Potential is a wonderful thing – but Manchester United need results now.