Mikel Arteta has warned Arsenal have to "dramatically improve" their defending at set-pieces and avoid conceding cheap goals to stay on track this season.
The Gunners were held to an entertaining 2-2 draw in their Europa League round of 16, first leg clash with Sporting Lisbon on Thursday.
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William Saliba opened the scoring with a 22nd-minute header from Fabio Vieira's corner before Sporting scored a similar goal at the other end 12 minutes later as Goncalo Inacio was left free to glance home Marcus Edwards' delivery.
Paulinho gave the home side the lead 10 minutes into the second half with a close-range finish before Hidemasa Morita suffered the misfortune of diverting Granit Xhaka's pass into his own net just after the hour mark.
Opponents have scored five goals against Arsenal from dead-ball situations since the World Cup, plus Bournemouth found the net with a routine straight from kick-off at Emirates Stadium last weekend.
And Arteta, who made six changes to his starting line-up with Sunday's Premier League trip to Fulham in mind, said: "We are conceding too many simple goals. We spoke about the importance of boxes, especially in a competition when you're in or out.
"Today we didn't defend that well enough and we have to be better at threatening the opponents as well. But of course you have an opponent there. We've played some really emotional games recently.
"We made a lot of changes today and sometimes that takes time to get that cohesion and you could feel it in the first 15 minutes that it needed some time to adapt and click. If we don't expose them though that's never going to happen.
"Sometimes the opponents are really good at what they do. When they get the delivery in the right space it's just half a centimetre sometimes whether you get that header out of the box or it goes in. It's true that it's something we must improve dramatically in the next few games if we want to keep winning."