Arsenal 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.
The Gunners are chasing their first Premier League title in 19 years, but manager Mikel Arteta still named a strong side.
William Saliba headed the visitors into the lead and Sporting equalised shortly after in near identical fashion, with Goncalo Inacio heading home Marcus Edwards' corner.
Former Tottenham midfielder Edwards then had a role in Sporting's second as Paulinho reacted quickest to finish inside the area.
Arsenal were level just after the hour mark, though, when Granit Xhaka's cross deflected off the unfortunate Hidemasa Morita and rebounded into his own net.
Arteta was unhappy with his side as they conceded two goals for the second straight game following their 3-2 Premier League win over Bournemouth.
"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 due 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 Qatar World Cup.
"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.
Sporting boss Ruben Amorim talked up Arsenal's quality on the eve of the contest but felt his side could have put themselves in a better position to advance into the quarter-finals after a strong showing.
"If you were to ask any Sporting fan before the match if a draw would be a good result, they would say 'yes'," he said.
"Now taking into account the 90 minutes, some of them will be disappointed. Our players were very smart, when recovering the ball we had quality in possession and created chances.
"I would say we could have won the match today. We could have scored (to make it) 3-1 and right after we conceded a second goal."