Alex Oxlade-Chamberlain’s second-half goal earned the Gunners a point against their local rivals after Nacer Chadli’s opener at Emirates Stadium.
The draw left Arsenal fourth in the Premier League table with 10 points from their opening six games this season.
And defender Mertesacker concedes that Arsene Wenger’s side will have to improve in the coming weeks.
“We know that we have good team spirit in the dressing room so we have to show it on the pitch,” said Mertesacker.
“Today we showed it in a good way but I think we can do better, much better. Overall it was a good performance and I think confidence is getting back.”
The World Cup winner added: “We lost two key players in the first half and it was more than a blow.
“We fought until the end, especially in the second half when they dropped.
“When they took the lead it was very difficult but we came back in a good way, not in a remarkable way. It could have been a bit more but overall they deserved a point because they defended well.”
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