On Wednesday evening, Manchester United looked nearly unbeatable; a Marcus Rashford hat-trick inspiring the Red Devils to a 5-0 thumping of Bundesliga leaders RB Leipzig.
Back in domestic competition, however, United faltered against a well-structured Arsenal side, who claimed a narrow three points at Old Trafford thanks to a penalty from Pierre-Emerick Aubameyang.
The spot-kick, taken in the sixty-eighth minute, was given away cheaply by Paul Pogba, who kicked out at the achilles of Hector Bellerin, leaving referee Mike Dean no choice but to award the Gunners a penalty.
And for the second consecutive week, a superb midweek from Manchester United was followed by a lacklustre performance at the weekend, with real questions being asked about where the Red Devils go from here.
Story of the game
Neither side started the game with particular emphasis on attacking; it was a cagey first five minutes for both Manchester United and Arsenal.
Both sides had decided to stick with their formations from earlier games; United with the diamond in midfield that had proved so effective against RB Leipzig, while Arsenal retained their three-man defence despite the absence of David Luiz.
It would be the visitors who claimed the first clear opportunity of the match after United had given possession away in their own half. Thomas Partey and Bukayo Saka linked up in the United area, but midfielder Fred was on-hand for the hosts to get the ball away from danger.
United were close to going behind after a quarter of an hour, as a neat, low ball in from Hector Bellerin nearly found Pierre-Emerick Aubameyang at the back post, but the Gabon international was inches away from sliding home at the far post.
The first real chance for the hosts came just after the twenty-minute mark, as Wednesday's hat-trick hero Marcus Rashford picked out Mason Greenwood on the left-hand side, but his first-time shot was straight at Bernd Leno from a narrow angle.
A pair of bookings for either side were the only real chances of note in the next ten minutes, as Fred and Aubameyang were cautioned for late challenges.
United's worrying first half continued when Alexandre Lacazette came close to finding the net after a lay-off from Aubameyang, but Victor Lindelof was there to block the shot, with Fred clearing away for the Red Devils. The visitors were nearly in front a moment later, as a one-two between Aubameyang and Willian resulted in the latter's dipped shot rattling off the crossbar.
With a minute until the break, Arsenal had a free header in the box, as Lindelof lost his marker in the box, and Saka's header narrowly cleared the bar.
However, the break would come and go with neither side ahead.
Aaron Wan-Bissaka would come close soon after the interval, as Greenwood's superb run into the area saw him tee-up the right-back, but his effort was blocked by Saka.
Five minutes into the second period, Lacazette found Aubameyang on the counter-attack as Arsenal out-manned United on the break, but the latter's shot just went past the post.
Harry Maguire would have a free header from a United free-kick a moment later, but his effort just went past the back post.
Thomas Partey had a long-range shot just before the hour mark after shrugging off the challenge of Fred; a calm save from David de Gea prevented the visitors from taking the lead.
Arsenal were awarded a penalty with twenty-two minutes remaining as Paul Pogba kicked the achilles of Bellerin, with the Gunners earning a deserved spot-kick.
Aubameyang stepped up and fired into the bottom-right corner; David de Gea diving the wrong way.
United's response to Arsenal taking the lead was lacklustre; a pair of substitutions saw Donny van de Beek and Edinson Cavani come on - but the hosts did not look like scoring as the last fifteen minutes beckoned.
The Red Devils hit the post with six minutes to go, as substitute Van de Beek's deflected effort smacked off Bernd Leno's near post.
And United's last chance to snatch a late point was in the hands of Nemanja Matic, whose shot from the edge of the box was poorly-struck and straight at Leno.
But that would be it; United would fall to yet another defeat at Old Trafford as their domestic campaign continues to stumble out of the blocks - a defeat to mark the one-hundredth match in charge for Ole Gunnar Solskjaer.