Arsenal manager Mikel Arteta urged his team to improve their defence after they had to fight back to earn a 2-2 draw with Sporting Lisbon in the first leg of their Europa League last-16 tie on Thursday.
Arsenal went ahead in the 22nd minute through William Saliba but Goncalo Inacio equalised in the 34th minute before Paulinho made it 2-1 in the 55th. The visitors finally levelled terms when Hidemasa Morita netted an own goal in the 62nd minute.
Arteta was unhappy with the north London side as they conceded two goals for the second successive game following their 3-2 Premier League win over Bournemouth last weekend.
"We are conceding too many simple goals and we've talked about the importance of boxes, and especially in a competition where you are in or out," Arteta told reporters.
"Today we didn't defend that well enough, and we have to do better up against our opponents."
Arteta said the team lacked their usual attacking threat owing to the absence of some key players, including striker Gabriel Jesus who is yet to reach full fitness after suffering a knee injury at the World Cup in early December.
"We had some positives because we showed a lot of resilience to get back in the game, but we need to defend our box much better and we have to be in the return leg to beat them," Arteta added.
The second leg will be played at the Emirates on March 16.
Premier League leaders Arsenal, who have a five-point lead over second-placed Manchester City, will travel to Fulham for Sunday's league game.