Mertesacker was full of praise for Thomas Vermaelen's understudy, who was making his first league start of the season.
The German told Sky Sports 1: "We had to be strong today, it was a very difficult match for us.
"We have a great squad this year and a lot of great centre-backs at the moment.
"Thomas is at home a little bit ill and Laurent came in and did fantastically. It was a solution.
"We are a strong squad and we have to stick together. We did well today and we have to keep on going."