Arsenal have kept clean sheets in four of the last six games that pair have started together and they were again impressive in the 1-0 victory at Craven Cottage on Saturday.
We were on a run of good results and good stability and we don’t concede goals at the moment so that’s why I keep them [Mertesacker and Koscielny] together
Wenger is happy with his defenders’ form but concedes it has been difficult to leave club captain Thomas Vermaelen out of the side.
“They’ve done well, I must say,” he said. “Vermalen had a very good game against Norwich but we were on a run of good results and good stability and we don’t concede goals at the moment so that’s why I keep them [Mertesacker and Koscielny] together.
"It’s a very difficult decision. Thomas is one of the best defenders in England, like the two others. I told them at the start of the season we have three outstanding central defenders.
“It’s about game by game that I make a decision, and it is about partnerships as well.
“He [Vermaelen] has been a substitute for quite a few games and he still plays a big part in the game. Koscielny was a long, long time during the season in that position and that can change very quickly.”Copyright 2016 The Arsenal Football Club plc. Permission to use quotations from this article is granted subject to appropriate credit being given to www.arsenal.com as the source