Manchester United manager Ruben Amorim put his first Premier League defeat since taking charge down to their set-piece shortcomings.
United were beaten 2-0 away to Arsenal on Wednesday, having initially held out through a goalless first half.
They were eventually undone by two corner routines, while the hosts came close to adding a third in similar fashion through Mikel Merino.
Amorim's side, meanwhile, failed to convert their own set-piece chances after seeing a Matthijs de Ligt header brilliantly saved by David Raya.
"The set-pieces changed the game, we could have been more aggressive towards the Arsenal box. Until the set-pieces, the game didn't have too many opportunities for both sides, the set-pieces killed the game," Amorim told BBC Match of the Day.
"They can put a lot of players near the goalkeeper and it's almost impossible to fight for the ball, but we have to manage to defend them and we already know we have to be better.
"We were committed and we lost to the set-pieces. If Matthijs can score in that moment, the momentum would be different.
"We tried to play, but they are a very organised team and it's hard to score. I felt the players were in control of the game, the set-pieces changed the game in the second half."
Manchester United defender De Ligt was in agreement with his manager, suggesting they performed well defensively aside from the goals conceded.
"In the defensive phase, the amount of chances we conceded without the set-plays [was good]," he told MUTV. "In the end, we played a little bit more offensive and conceded some chances at the end.
"But in general, we have to – and we will – offensively improve. We need to know what everyone is doing and we could have hurt them much more.
"We're still quite early with the new coach, but there are still positives. Overall, I'm really disappointed with the lack of points."
Their trip to the Emirates Stadium marked Amorim's fourth game in charge in all competitions. De Ligt is conscious that he has had little time and is sure improvements will come.
"We're on the road to improve as a team and I think the manager said it also. Maybe if we do it a certain way we have a bigger chance of getting results right now," he said.
"I think the way we are going now is obviously not perfect yet and everybody has to improve a lot and get used to the new system, but you can see certain things that are improving."
Simeone wary of Champions League challenges despite winning run
Diego Simeone hopes Atletico Madrid can continue their excellent recent form to clinch a top-eight spot in the Champions League, but he remains wary of the challenges ahead.
Atletico won a 10th game in succession as they eased past Slovan Bratislava 3-1 at home on Wednesday, a result that leaves them one point outside the top eight with games against Bayer Leverkusen and RB Salzburg still to come.
It is Atletico's longest winning run in over a decade, since winning 13 in a row between August and October 2012.
"There are two games left, and they are very good teams," Simeone said. "We face Leverkusen, one of the best in the competition, and we will see how far we can go.
"We will give our maximum and see what the sum of our points brings us."
Julian Alvarez and Antoine Griezmann, who scored a double, eased Atletico to victory against Slovan, though Simeone was left to rue a conceded penalty that briefly sent jitters through his side as the visitors made it 2-1.
"The effort was there," he said of Wednesday's performance. "The first half was controlled, with important chances, and then in the second, it was also controlled beyond that penalty.
"The players managed it well, and we won a game in which we cared a lot about the three points."
Atletico have been magnificent on their 10-game winning run, scoring 30 goals in that streak, effectively putting their sluggish start to the campaign behind them.
"It's football and football turns, changes, with moments of difficulty and others that lead you to be very good. We know what we have [in the team], where we are going, and we always try to improve."
Wednesday's match was notable for the fact that at one stage both coaches had their sons on the pitch. Giuliano Simeone started for Atletico, while Slovan coach Vladimir Weiss brought on his son, also Vladimir, for the final six minutes.
"I think that if the coach [Weiss] has chosen to put the player on, he did not think of putting his son on. Like me, neither of us have looked at them as a son, we look at them as footballers," Simeone added.
Among teams who have scored 100+ goals in the Champions League, only Lionel Messi (20.4% for Barcelona) has scored a higher percentage of a club's goals in the competition than Griezmann has for Atletico (19% - 36/189).
All of his Champions League goals for the Rojiblancos have come under Simeone. Only Messi under Pep Guardiola (43) and Mohamed Salah under Jurgen Klopp (41) have scored more goals under the same manager in the competition.
"The Champions League is complicated, goals matter a lot," Griezmann said. "It is a shame about the penalty. In the second half, we let them have the ball a lot. These are things to improve as always, but we are on the right track."
Feyenoord 4-2 Sparta Prague: Quick start eases hosts to win
Two early goals laid the foundation for Feyenoord to boost their Champions League hopes with a comfortable 4-2 win over Sparta Prague on Wednesday.
Two goals in 90 seconds from Gernot Trauner and Igor Paixao gave Feyenoord a dream start, with Anis Hadj Moussa and Santiago Gimenez adding the goals that keep them on course for a spot in the knockout stages.
The Dutch hosts needed only eight minutes to get on the scoresheet from a corner with Trauner's downward header wrong-footed Peter Vindahl.
The second came when Feyenoord's pressing stripped Kaan Kairinen of possession and Paixao ran from the halfway line before bending the ball wide of the goalkeeper.
Hadj Moussa scored a stunner 30 minutes in, cutting inside from the right to fire a left-footed shot into the top corner. Feyenoord then failed to clear a long throw that was flicked onto Albion Rrahmani at the back post to make it 3-1 in the 43rd minute.
Gimenez restored the comfortable margin when he took advantage of a defensive slip to bundle the ball over from close range just after the hour mark, while Sparta's second goal came from a cross in the 79th minute which was steered home by a sliding Thomas Beelen into his own net.
Feyenoord moved on to 10 points, while Slavia suffered a fourth defeat in six group games and have a tough task to avoid elimination.
Data Debrief: A happy reunion
The match saw Brian Priske reunited with the team he took to the double in the Czech Republic last season, with his former assistant Lars Friis now in charge at Sparta.
Only Barcelona (five) have scored three or more goals in more different Champions League games this season than Feyenoord (four), who netted three first-half goals and four goals overall for the first time in the competition.
Having led 2-0 after just 10 minutes, it is the quickest two-goal lead Feyenoord have held in a major European game since September 1972.
Meanwhile, Sparta have failed to win any of their last 15 away games in the Champions League (D3 L12), the joint-longest ongoing such winless streak in the competition (level with AEK Athens, whose last game was in 2018).
Benfica 0-0 Bologna: Italian's side on brink of Champions League exit after Lisbon stalemate
Bologna are on the brink of elimination from the Champions League following a goalless draw with Benfica.
The stalemate at Estadio da Luz left the Serie A side on two points after their opening six matches, and six points adrift of the play-off places.
Benfica thought they had taken the lead after just two minutes, but Vangelis Pavlidis saw his effort ruled out for offside following a VAR review.
Bruno Lage's side continued to carry the greater attacking threat, registering 15 shots to their opponents' six.
Skorupski made a superb reflex save to deny Pavlidis after the break, and the hosts' profligacy later on meant they had to settle for a share of the spoils.
Data Debrief: Bologna draw another blank
There were 10 yellow cards brandished during the game, the most in any match in this season's Champions League.
In drawing another blank, Bologna are the third time in Champions League history to score as few as one goal across their first six games in the competition, after Galatasaray in 1993-94 and Levski Sofia in 2006-07 (both also one).
Meanwhile, Benfica recorded a 0-0 draw on home soil in the Champions League for the first time since December 2014, and have now failed to score in four of their last seven games in the competition at Estadio da Luz.
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