A subdued and somber Arsene Wenger was left to rue Arsenal's lack of efficiency after his team lost 2-1 at home to Manchester United on Saturday in a game it dominated.

The host took advantage of a makeshift United defense to pour forward in the first half and only the excellence of visiting goalkeeper David de Gea prevented the London side from being well ahead at halftime.

But the lack of ruthlessness and the naivety which has plagued Arsenal in recent years cost it dearly at the Emirates Stadium again as a second half own goal by Kieran Gibbs and Wayne Rooney's breakaway finish gave United the points.

"We have not dominated many games against United like we did today but we were not efficient enough and made defensive mistakes," Wenger said.

"We had plenty of opportunities to win the game and we are very disappointed. We were a bit naive."

Wenger's misery was compounded by injuries to midfielder Jack Wilshere and goalkeeper Wojciech Szczesny, who were both forced off in the second half.

"Jack has an ankle injury," Wenger said. "I don't know how bad it is yet. Wojciech is not bad."

As the home fans streamed out of the stadium straight after Rooney's goal, isolated shouts of "Wenger out!" could be heard in the north London gloom.

Having thrown away a 3-0 lead to draw 3-3 with Anderlecht in the Champions League and conceding two late goals to lose 2-1 at Swansea City in its previous league game, the Arsenal faithful are beginning to question Wenger's methods again.

The team has slipped to eighth in the table, 15 points behind leader Chelsea, and is already out of contention to win the title for the first time since 2004.

It hosts Group D leader Borussia Dortmund in the Champions League on Wednesday and Wenger needs a strong performance to lift morale.

"We have to be patient but there were a lot of positives today," Wenger said quietly, his body language suggesting a manager under pressure after 18 years in charge of his club.