Prior to last weekend’s showdown at Old Trafford, it was roundly stated that Manchester United was going to face its biggest challenge of the new campaign when Louis van Gaal’s side matched up with league leading Chelsea.

Following the contest, the majority of football experts agreed that United displayed their best performance of the early season by securing a last-minute draw against Jose Mourinho’s men.

Truth be told, the twenty-time champions of England benefited from facing a Chelsea side playing without their leading scorer, Diego Costa, as well as their versatile Brazilian midfielder Ramires and regular starting left-back Cesar Azpilicueta. United also profited from a questionable second yellow card which saw Blues defender Branislav Ivanovic sent off in the moments before the set piece that led to Robin van Persie’s equalizing goal.

That said, leading up to the match the consensus among Premier League experts was United were going to be thrashed by a Chelsea squad which had only dropped two points during their prior eight league matches. But Van Gaal’s men did a good job man-marking Jose Mourinho’s midfield; thus slowing down the production of Cesc Fabregas (largely through the efforts of a rejuvenated Marouane Fellaini). United also were able to keep a high defensive line because Chelsea’s 36-year-old striker Didier Drogba wasn’t capable of running beyond the opposition.

Now that the match with Chelsea is in the rear view mirror, United are staring at the task of slowing down a Manchester City attack which scored seven times against their defense last season; including a 4-1 demolition at the Etihad.

Yes, the setbacks to their Manchester neighbors came while United were under the failed leadership of former manager David Moyes. But the Scotsman was known for his ability to defend against bigger opponents while he was in charge of Everton. He also went into his first derby match with United having the luxury of being able to call on the club’s best centre-backing pairing: Rio Ferdinand and Nemanja Vidic.

But even the presence of that defensive duo couldn’t prevent the 4-1 lashing City dealt United at the Etihad in last season.

While City have sputtered recently during their domestic and European fixtures, make no mistake about it, Manuel Pellegrini’s attacking options of Sergio Aguero, Edin Dzeko, David Silva (pending his fitness), Jesus Navas, Samir Nasri, Yaya Toure and Stevan Jovetic will present Louis van Gaal’s ‘work in progress’ defense with its biggest challenge to date.

Despite being “in a difficult moment” and suffering from “a lack of trust”, according to Manuel Pellegrini, City can be expected to put in its strongest shift of the season against United on Sunday. The Etihad will be at full voice hours before kick-off and the Citizens will be energized by the arrival of their cross-city rivals.

Injuries, suspensions, poor form and a lack of available centre-backs during the summer window have left Louis van Gaal with a continuous selection problem in defense. United’s returning defenders: Chris Smalling, Phil Jones and Jonny Evans have suffered from recurring injuries and have yet to show the leadership necessary to led the club’s back line.

Marcos Rojo has seen the most consistent playing time since his work visa issues were resolved, but has displayed up-and-down performances at best. It appears the defender is adjusting from being used predominantly as a left-back at Sporting Lisbon to a left-sided central defender under Van Gaal.

While, Luke Shaw has only recently returned to the lineup since suffering an injury during the club’s pre-season tour of the United States. Though the 19-year-old put in a solid shift against his boyhood club Chelsea this past Sunday.

Van Gaal has stuck with a combination of Shaw and Rojo on the left-side of United’s defense, with Rafael appearing to be the Dutch boss’ choice at right-back – since the manager has no other options at the position; outside of using Chris Smalling and Antonio Valencia.

United’s teenage replacements Paddy McNair and Tyler Blackett showed promise when they were thrust into the club’s starting eleven. But suspension, injury and Van Gaal’s preference to start left-footed defenders in the two left-sided defensive positions (also right-footed defenders on the right side) have ruled the youth academy products out of the lineup in recent matches.

The issue for United has been at the right centre-back position. Smalling, Jones and/or Evans have yet to stake their claim to the position to this point. Whether it’s due to injury or simply an inability to lead the line, these returners have seen limited time on the pitch and been a major disappointment when they have been called on.

Van Gaal has used eight different centre-backing pairings thus far in United’s opening nine BPL matches.

The manager has been quick to deflect attention away from his weakness in central defense by pointing out that United need to defend better as a team.

“I want my defensive organization to be better but the defense begins in the striker’s position,” the Dutchman said in early October. “It’s teamwork, so when we concede a goal it’s not only because of an individual error from the defense. It starts somewhere else and that is what we have to analyze and improve.”

“All the teams are mostly scoring goals out of counterattacks against us and free-kicks and corners – not that they make their goals like we do out of building up, first, second, third and fourth phase.”

“That’s the difference and, when you want to be a big team, you have to perform like that. You must dominate the game. When they are organized and also when the team is organized in their own half, then you need these kind of players [who can carry this out]. But you also need to think about the balance of the team and the defensive organization at the same time. That is what I’m working on.”

While there is a great deal of truth in Van Gaal’s statement, it can’t be overlooked that United’s defense has been prone to mental lapses and has shown a lack of leadership this season. Now more than ever the club is in need of Premier League-hardened defenders, but have been left with a unit made up of players who – to this point – have shown that they may not be up to the task.

Daley Blind has arrived from Ajax and has made every effort to be United’s vocal leader while operating in front of the back four. The versatile midfielder has had a promising start to his career at Old Trafford. But the club lacks players who can match Blind’s tactical awareness and leadership skills within its back four.

It can be expected that Manchester City will put United’s defense under serious duress during the opening minutes of their encounter this weekend at the Etihad. Manuel Pellegrini will want to see his side duplicate the effort which saw City score four times in the opening 50 minutes of last season’s contest between the two Manchester clubs at the Etihad.

The belief among football pundits is that United will be involved in a number of matches this season where the club will need to outscore its opponents, due to their defensive deficiencies. But Louis van Gaal is not the type of manager who would allow his side to throw out his defensive principles and get involved in a shootout. The manager will continue preaching his philosophy and providing every player at every level of the club an opportunity to prove they can compete in the Premier League and eventually Europe again.

According to reports, United is returning more players from injury; and Van Gaal should have a wealth of attacking options to choose from against City as Radamel Falcao, Ander Herrera and Wayne Rooney return to 100% fitness and suspension respectively. The manager’s selection of defenders will also increase, with Phil Jones being the only major doubt for Sunday’s encounter.

Van Gaal may also go with the option of pairing Daley Blind with a recovered Michael Carrick in front of the back four. The 33-year-old has been on United’s bench for its past two BPL matches and played 64 minutes for the club’s under-21 squad last Friday as Van Gaal slowly works the midfielder back to match fitness following surgery that kept him out of United’s entire preseason.

As big a test as league-leading Chelsea were last Sunday, Manchester City’s attack will present United’s defenders with a completely different set of challenges to overcome this weekend.

Will the club continue to develop as a cohesive unit and build on the momentum of its dramatic last minute equalizer against Jose Mourinho’s side? Or will United be brought down to Earth and reminded that they are still a few more defensive signings away from being serious top four challengers this season?

Former Manchester United and Manchester City goalkeeper Peter Schmeichel gives his thoughts on this weekend’s derby on the BBC’s Focus Forum: