Arsenal boss Arsene Wenger admits it's been difficult freezing out Thomas Vermaelen.
The Belgian has started just one game this season.
Wenger said: "It is unfortunately part of our job, that sometimes we frustrate people. I can understand completely his frustration.
"He is rightly frustrated because he has the quality to play, because he behaves with a fantastic attitude and is a credit to our job, not everybody behaves like him, it is absolutely fantastic.
"They [Mertesacker and Koscielny] have done well, and there was no particular reason to change.
"His time will come. In our job the most important is to be ready and he behaves like somebody who will be ready when needed."
