The rivalry that once dominated the Premier League has been reduced to a sideshow for the best part of two decades due to the respective decline of both teams -- but Sunday's clash at Old Trafford proved that two of England's biggest clubs are, after a torturous wait, on the way back.
But now is the time for realism and patience. United and Arsenal are two clubs with a proud history of success -- between 1996 and 2004, no other club won the Premier League -- but neither will win the title this season.
For the first time in longer than both clubs care to remember, though, a return to glory is now visibly on the horizon. Come back in two years' time and United and Arsenal could be the big two once again.
United's 3-1 victory on Sunday, inspired by midfielder Christian Eriksen and sealed by two-goal striker Marcus Rashford, ended Arsenal's 100% winning start to the season and extended the remarkable revival by Erik ten Hag's team, which is now in fifth position, just three points behind Mikel Arteta's Gunners, at the top of the table.
After two games, United were bottom of the table on zero points after Ten Hag became the first Old Trafford boss in 101 years to suffer two defeats at the start of his reign -- but four straight wins later, belief and confidence are running through his team and the club's supporters. United fans still want rid of the Glazer family, the club's owners, but the negativity that has hung over Old Trafford like a thunderstorm for years has been lifted, and beating Arsenal means that genuine optimism has returned to the red half of Manchester.
Arsenal are further down the track than Ten Hag's United in terms of rebuilding confidence and are giving their fans hope that success can finally return to the Emirates.
Arteta has been backed by the club's owners to make and take some tough decisions on the makeup of his squad, including cancelling the contracts of high-earning but under-performing stars such as Mesut Ozil and Pierre-Emerick Aubameyang. Pep Guardiola's former assistant at Manchester City has given youth a chance, backing the likes of Bukayo Saka, Emile Smith Rowe and Gabriel Martinelli, and the summer additions of Gabriel Jesus and Oleksandr Zinchenko, in particular, have fast-tracked the injection of a winning mentality into the squad.
Missing out on the Champions League with a disastrous end to the past season highlighted the work still to be done by Arteta, but five straight wins at the start of the campaign have highlighted the progress made by the manager, and the team now has a platform on which to build a sustained push for a top-four finish.
Manchester City and Liverpool are still some distance ahead of United and Arsenal in terms of quality and experience of winning, but they will both have noted the upsurge at Old Trafford and the Emirates in recent weeks.
Last season's top two teams also know that Arsenal and Man United will still be troubled by inconsistency.
Arsenal are a young team and have yet to learn how to go a place like Old Trafford, kill the game and come away with a victory. United, meanwhile, still lack quality in depth and need to boost their firepower, having scored only eight goals in six games so far.
"I understand fans are dreaming, the standards of Man United have to be good," Ten Hag said. "We're at the start of a process, we're still far away. We have to get doing things much better than we do. It's a process -- you have to lift the standards every day."
Summer signings Lisandro Martinez, Christian Eriksen, Tyrell Malacia, Casemiro and Antony, who scored on his debut against Arsenal, have added much-needed character and tenacity to the squad, however, and Ten Hag has brought tactical expertise and leadership and has already improved the likes of Jadon Sancho, Fred and Rashford.
United have positive momentum now, but there is also a different feel to Arsenal, who have been plagued by fragility since the final days of Arsene Wenger's reign in charge.
The defeat at Old Trafford wasn't caused by a lack of Arsenal character, rather a failure to capitalise on a period of dominance and United being more ruthless in front of goal. There is no sense that Arteta's players will go into a tailspin after losing to United, and that is a testament to the improvement at the Emirates.
"For these boys to play the way they did here, big credit to them, but we need a better result," Arteta said. "We have to learn that you need to have so much discipline in these games."
It's been almost 20 years now since United against Arsenal was the game that mattered most in the Premier League.
Back in September 2003, the two teams played out a 0-0 draw that became known as the Battle of Old Trafford because the brooding tensions between the two teams spilled over into a postmatch brawl. That season, an unbeaten Arsenal won the league for the last time, becoming "The Invincibles" in the process, but they have never come close to winning it since.
It will be 10 years next May since United last won the title, and they have also become title no-hopers since Sir Alex Ferguson retired as manager.
Yet finally, United and Arsenal look to be on an upward trajectory again, and that's good news for the Premier League.