Arsenal manager Arsene Wenger backed Alexandre Lacazette despite taking the forward off in the second half of his side's draw with Chelsea.
The France international has made a decent start to life in England, scoring two goals in six matches in all competitions since arriving for EUR53million from Lyon.
But Lacazette, 26, was unable to have an impact in the 0-0 draw at Stamford Bridge before being substituted in the 66th minute, the forward missing a glorious chance prior to half-time.
Wenger praised the forward and said with the likes of Alexis Sanchez, Olivier Giroud and Theo Walcott on the bench, he was always going to make a change in the derby clash.
"I felt Lacazette had a very good first half. He worked very hard for the team, he's very disciplined and it takes him some time to maintain that intensity for 90 minutes," he said.
"Don't forget as well I had Walcott, Giroud and Alexis on the bench, so after 65 minutes in a game of that intensity, it's normal that you bring some fresh legs in."
However, Wenger said he would never ask a striker to get used to being substituted.
"I never ask any striker to understand that he has to make room for somebody else," he said. "You can never do that."