A defensive lapse at a corner proved costly for Arsenal in their defeat by Manchester United and they could be forgiven for feeling unfortunate given the illness which had disrupted their team before kick-off in one of the biggest games of the season. But the disappointing performance of Olivier Giroud was also a major factor in the loss at Old Trafford and will leave the visitors with a sense of what might have been.
The familiar figure of Robin van Persie, the predecessor who Arsenal hoped Giroud could replace, was the match-winner when he took advantage of uncertainty to head the only goal of the game in the 27th minute. It means Van Persie has now scored in the last five Premier League games between these two clubs and it helped United to fifth in the table.
Regardless of Arsenal's questionable defending at the set-piece, it was a superb header by the Dutchman to secure David Moyes' finest hour since succeeding Sir Alex Ferguson in the summer. But it was otherwise far from a classically entertaining game as the teams managed a combined total of just four shots on target. It demonstrated the fine margins which define games between the clubs in England's top flight, with only six points now separating the top eight.
But Arsene Wenger's Arsenal, who beat Liverpool and Borussia Dortmund in their two previous games in a gruelling run of matches which would stretch any team, will be left feeling frustrated. They lost key men Per Mertesacker and Tomas Rosicky to a bug just hours before kick-off and their lead at the top of the table is now only two points. However, there was still a sense - particularly with Giroud - of a missed opportunity for what would have been just a second win in 10 league visits to United.
United's Van Persie missed a penalty against Real Sociedad in midweek having come on as a substitute but he started up front with Wayne Rooney in a side also containing Jonny Evans, Phil Jones, starting in central midfield, and Michael Carrick.
United were desperate to claw back Arsenal's advantage in the table, which stood at eight points before kick-off, and they were assisted by the absences of Mertesacker and Rosicky, who had both fallen victim to illness on the morning of the game.
Mertesacker's absence looked an especially big setback given his good form this season and the way he had led the Arsenal back four. He was replaced by Thomas Vermaelen and it meant Laurent Koscielny had to switch to the right centre-back.
Elsewhere in Arsenal's team, Mathieu Flamini returned in place of Rosicky, which led to Aaron Ramsey having to go out of position to right midfield. Jack Wilshere was only fit enough for the bench.
Story of the game
Sky Sports' Gary Neville asked shortly after Van Persie scored: "How many times in big games, tight games, do set-pieces become an important factor?" Such was the case at Old Trafford and Wenger will be full of regret. It was only the second goal Arsenal have conceded from a corner in the league this season and it was impossible to ignore that Van Persie scored his technically-superb header from an area - the near post corner of the six-yard box - in which the absent, giant Mertesacker would usually be defending.
In Arsenal's zonal-marking system, Giroud seemed to be filling in for Mertesacker in the near post area but he could not get to the ball. Ramsey had been drawn under play in the delivery and Kieran Gibbs came off the back post where the ball passed on its way into the back of the net. To compound Arsenal's individual errors, it appeared a tactical decision had also been made not to set up any blockers on the penalty spot to prevent Van Persie having a free run at the ball from his starting position on the edge of the 18-yard area.
Rooney assisted the goal when taking the corner and was named man of the match having shown superb work-rate in covering 11.1 kilometres - a total only bettered by Ramsey - to continue his fine season, although his pass completion rate was still only 79.4 per cent and he lost possession 22 times. The England international after the final whistle revealed United had been working on their set-pieces, because of a perceived lack of height in Arsenal's team.
It was otherwise a tense and somewhat scrappy game. Rooney was the only player to have had a shot in the opening 20 minutes while Arsenal had failed to have an effort on target from two attempts before half-time. Arsenal's negativity at Old Trafford is a recurring theme given they failed to have a shot on target until the 92nd minute when they visited in November 2012.
Moyes had told Sky Sports before kick-off: "The middle of the pitch is somewhere we have to try and make sure we are solid. Arsenal have a lot of midfield players who can hurt you through that area. We have to make sure we do that right. I think, then, if we can get the ball in wide areas at the right times, we can try and cause them problems."
That is exactly what United tried to do and they had 13 crosses from open play in the first half to Arsenal's seven. Another problem for Arsenal, who had a lack of pace on the counter-attack, was Giroud's failure to hold the ball up. It was the Frenchman who gave the ball away in the build-up to the corner for Van Persie's goal and he lost possession on 27 occasions during the game. That was a higher total than any other player.
Arsenal improved after half-time, when Wenger had clearly instructed his players to make greater use of the wide areas. Ramsey began the second half hugging the right touchline. Bacary Sagna also got much further forward from right-back and that meant Arsenal had 12 crosses in the second half compared to that total of seven before the break. United had perhaps been weakened by the loss of Nemanja Vidic to a head injury at half-time which saw Jones drop back into defence.
Wilshere also came on for Flamini in central midfield in the 61st minute, with Ozil going out to the right and Ramsey moving centrally, and that again brought a greater threat from Arsenal. In the second half, they improved their possession (56% in the first half and 64.4% in the second half), total passes (279 in the first half and 262 in the second half) and passing accuracy (82.8% in the first half and 79% in the second half).
But United, in front of the watching Ferguson in the stands, largely kept Arsenal at arm's length. Patrice Evra was especially important in this alongside Chris Smalling when making a joint game-high total of six tackles each while the Frenchman also completed a higher number of clearances (11) than any other player on the pitch.
This, combined with Giroud's disappointing game - winning only 25% of his 20 individual duels and failing to hit the target with his two shots, meant Arsenal, despite their dominance in the closing stages, could only match United's total of two shots on target throughout the game. The set-piece cost Arsenal and they will now have to react in a positive manner when they face the difficult game of hosting in-form Southampton after the international break.
"We have not had many good ones but this was a big result for us today. Arsenal have been in terrific form this season and we had to play well to get the result. I thought we started the game well. We were very good in the opening 20-30 minutes in the game. I thought our whole determination, the effort, commitment and good play helped us in the second half when we were under more pressure."
"It is difficult to lose the game the way we did. But, overall, to sum it up, we conceded a goal to a set-piece and in a big game that can make the difference. In the first half, we were too nervous and rushed our game too much, and lost too many balls and did not track the ball. On the corner, the goal came from one of these balls."
"Rooney is the reason why they are only five points behind this season, because Manchester United have been poor. It is not the vintage United we have seen of old and there are a lot of players who have not performed. But he is the one who has dragged United up there, virtually single-handedly at times."