David Luiz has suggested Arsenal were not fit enough under former boss Unai Emery.
Arsenal picked up their first win of Mikel Arteta's regime as they beat Manchester United 2-0 at the Emirates.
The Gunners were excellent in the first-half but their efforts showed as they tired in the second half, as they did against Chelsea on Sunday.
But while they couldn't hold on in the London derby, they were able to do so against United to climb back into the top half of the table.
Luiz was excellent in central defence but admits Arsenal's players are struggling to cope with Arteta's demands.
"I think it was great. We were great in the first half and showed a maturity in the second," Luiz told BT Sport.
"We have to be honest and admit physically we are not ready, but you have to show heart and this team is ready to change a lot of things.
"We showed that today and we are going to do big things in the future."
Arsenal's players were visibly delighted at full-time as they were congratulated by Arteta and Luiz lauded the new boss.
"Mikel Arteta is a great coach, he was a great player. I think he can improve every player," he added.
"And in life when you are happy results can be different. If you work with happiness and believing what you are doing it is totally different. Our season started very bad but things can change."
