On Thursday, Arsene Wenger marked 19 years since taking the reins at Arsenal. He is by a considerable distance the longest-serving manager among Europe’s top leagues, and his longevity and consistency is worthy of considerable praise. Arsenal have qualified for the Champions League for 18 successive seasons and for the past 15 have made it past the group phase. Yet it is the familiarity of Arsenal under Wenger that is once again bringing mounting frustration ahead of Sunday’s clash with the Frenchman’s greatest rivals in his time at the club, Manchester United.

Tuesday’s Champions League defeat to Olympiakos wasn’t just a huge blow to Arsenal’s hopes of making it to the knockout phase; it was a story that has unfolded so many times before. Under pressure to deliver a result against opposition they were expected to beat, Arsenal first began with an almost complacency that their talent would see them through. Once that certainty was threatened, anxiety then kicked in, leading to their efforts to make amends being undermined by a level of naivety staggering to witness at a club that has almost everything in place to be up there with the world’s best. It was last season’s embarrassing capitulation to Monaco in the Round of 16 all over again.

It is now already more than 11 years since Arsenal lifted one of the two titles by which clubs of their stature are defined -- the domestic league and the Champions League. Despite the financial constraints now long behind them, the north London club look no closer to ending that streak. Indeed, the Champions League this season may now see them go backward after failing to take a point from their opening two matches.

In contrast, their next opponents, who have exposed Arsenal’s flaws as much as any other in the past decade, do look to have taken a step forward. Manchester United enter Sunday’s Emirates Stadium showdown sitting top of the Premier League -- three points and three places above Arsenal -- for the first time since the retirement of their own legendary manager Sir Alex Ferguson.

In midweek, they, like Arsenal, were under pressure at home after losing their opening Champions League contest. Louis van Gaal’s side, however, responded with a 2-1 victory that was not overwhelmingly impressive, but never the less impressively efficient.

Still far from the finished article, United have far more weaknesses in the squad than there should be at a club that has spent close to £250 million since Van Gaal took the helm 15 months ago. But, with four successive wins to their name, momentum is building. Crucial to that upturn has been the most scrutinized signing of Van Gaal’s reign.

Anthony Martial had made just 29 league starts before Manchester United made him the world’s most expensive teenager, committing an initial £36 million to bring him in from Monaco at the end of the transfer window. Even his biggest admirers at Old Trafford could surely not have envisaged quite the immediate impact he would make.

There is of course the composure of his finishing, which has seen him score four goals already for his new club. But his influence goes far beyond the goals, and stretches across the whole of his new team. That was evident against Wolfsburg, when, after going behind, Martial’s pace and movement leading the line considerably aided United turning around the contest with last season’s Bundesliga runners-up. In comparison to the lumbering Wayne Rooney, Martial’s runs opened up space for Juan Mata’s creation to decisively come to the fore.

But Van Gaal will know truer tests are about to come. October sees Manchester United visit Everton, Crystal Palace and CSKA Moscow as well as hosting title favorites and local foes Manchester City. But first comes a trip to the Emirates Stadium, and a chance to avenge one of the worst performances during Van Gaal’s reign, when Arsenal triumphed in March’s FA Cup quarterfinal.

Prediction: The momentum is with United, but it would be wrong to write off Arsenal. It is after all another familiar trait of the Gunners under Wenger that they respond from disappointment, ensuring that no matter the rollercoaster ride during the season the end result remains roughly the same. The big question lies in how United will approach the fixture. Will Van Gaal continue to focus on controlling the match, or will he take a pragmatic approach as he did when beating Arsenal in the corresponding Premier League fixture last season? Should they sit back and hit Arsenal on the break with the pace of Martial then United are surely favorites. But their form may lead Van Gaal to take a more ambitious approach and that could play into Arsenal’s hands to allow them to claim a draw.

Predicted score: Arsenal 1-1 Manchester United

Team news

Laurent Koscielny looks set to miss out after suffering a hamstring injury in the defeat to Olympiakos, while there are doubts over Mathieu Flamini and Mikel Arteta. Jack Wilshere, Danny Welbeck and Tomas Rosicky remain long-term absentees.

Manchester United: Van Gaal has revealed that Michael Carrick will be fit to return after sitting out the victory over Wolfsburg with a slight injury. Ander Herrera should also be back available, but Marcos Rojo and Luke Shaw will again miss out.