Unai Emery was disappointed his Arsenal side conceded late on in their 4-2 Europa League victory over Vorskla Poltava.
Pierre-Emerick Aubameyang scored twice while Danny Welbeck and Mesut Ozil also got on the scoresheet to put Arsenal 4-0 in front at the Emirates, but the Ukrainian side denied Bernd Leno a clean sheet on debut.
After conceding an injury-time goal in the 2-1 victory over Newcastle at the weekend, Emery was disappointed to again see his side's defence breached late on.
He said: "It is our disappointment today... We need to continue to improve in this because the last minute at Newcastle and today, in the last 20 minutes, there were two goals, and our challenge is to be competitive for 90 minutes each match and it is one thing to improve."
The Spaniard was, however, pleased to have begun his first European campaign at the club with a victory in front of the home fans.
"We are happy because the first match with our supporters is very important to win and also, at the beginning of this competition, to take the confidence and to take the first victories," he added.
"I thought the first 60 minutes were very good, there was good progression in the match and imposing our ideas against theirs. With the four goals, we are thinking in a good moment but we want to be competitive over 90 minutes.
"We are speaking before each match to take a mentality of 'don't concede chances easily'. I think today is a good opportunity for not accepting goals, but the last 30 minutes, we needed to be better."
Emery handed Lucas Torreira and Bernd Leno their first starts at the Emirates and indicated that the goalkeeper will get more game time in the coming weeks, when Arsenal will play games every few days.
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"Lucas is playing with a passion and [it has a positive effect] on the team and on our ideas. Each minute he is playing, he is doing very well," he added.
"And for Leno, it's the first match also. I think that we conceded two goals but not [because of] mistakes. For him, it [provides] minutes and confidence [which are important] because he will be continuing to play matches in the next weeks."