Manchester United's inconsistent start to the Premier League season faces its biggest test yet on Sunday when they host Manchester City, with the derby offering Michael Carrick's side an early opportunity to show whether they can mount a genuine challenge at the top.City arrive at Old Trafford with a perfect record under new manager Enzo Maresca, although their performances have suggested the team is still adjusting to life under Pep Guardiola's successor.Also Read: UEFA urges US court to reject FIFA subsidiaries’ bid to intervene in Infantino caseThe first Manchester derby of the season could therefore provide a revealing snapshot of where both sides stand.Carrick's influence at United was immediate last season after he took over as interim manager following the club's decision to part ways with Ruben Amorim in January. His tenure began with a 2-0 home win over City and ultimately delivered 12 victories from 17 Premier League games, with just two defeats, as United finished third and secured a return to the Champions League.This season, however, the optimism has been tempered by an unusually quiet transfer window. Youri Tielemans and Andrey Santos were the club's main signings, before United suffered an opening-day 2-0 defeat to newly promoted Hull City.Bruno Fernandes then produced a hat-trick to inspire a 5-2 comeback against Ipswich Town, but United again dropped points last weekend after conceding a stoppage-time equaliser in a 2-2 draw at Everton.United's defensive concernsFormer United defender Gary Neville has singled out the team's defensive frailties, warning that they could become a defining weakness unless addressed."I don't think it's going to go away and that's my big fear," the Sky Sports analyst said this week.City's visit will offer a particularly stern examination, with Erling Haaland in prolific form. The Norway striker scored both goals in Tuesday's 2-0 Champions League win at Porto, taking his tally to five goals in three games this season.Maresca could hardly have asked for a better start to a job widely regarded as 'mission impossible' because of the challenge of succeeding Guardiola.City's substantial spending during the close season has given the new manager a deep squad, although finding the right balance among his new options could take time."I'm very happy," Maresca said after City's battling 1-0 win over promoted Coventry City. "Also, because I'm getting to know the players day by day, we are getting to know each other.Also Read: Mbappe, Haaland fire as Real Madrid and Manchester City open Champions League with wins"That means we are going to be better and better."Arsenal set the early paceFor City and the rest of the title contenders, Arsenal's start will be an additional concern.The champions have won all three of their Premier League matches and followed that up with a hard-fought 1-0 Champions League victory at Napoli on Wednesday. Mikel Arteta's side have carried the momentum and stability of last season into the new campaign.Arsenal travel to Sunderland on Saturday, a fixture that could prove less straightforward than it appears after they were held to a 2-2 draw at the Stadium of Light last season.Liverpool, meanwhile, took some time to find their rhythm under Andoni Iraola. Successive victories at Ipswich and Atletico Madrid in the Champions League have changed the mood, and they will now welcome Fulham, managed by former Liverpool player Alvaro Arbeloa.Arbeloa has endured a difficult start, with three straight defeats and seven goals conceded making him the first manager this season to come under significant pressure.Tottenham Hotspur manager Roberto de Zerbi could soon find himself in a similar position. Despite spending £300 million ($405.03 million) during the close season, Spurs are still without a win or a goal after three matches.Just over three months ago, the highly rated Italian completed what had looked like a rescue mission at Tottenham, guiding them to a final-day victory over Everton that secured their top-flight status.Everton return as visitors on Saturday. While the stakes are no longer quite as high, De Zerbi will need a victory to start showing that Tottenham's heavy investment can translate into results.