The Denmark international was on hand to slam home from close range in the 89th minute before Theo Walcott sealed the three points moments later.
Mertesacker stressed the importance of having a second forward who can make a telling contribution from the bench.
It's important to have a second proper striker who comes in and just does what he usually doesPer Mertesacker
"It was a vital goal from Nicklas," the defender told Arsenal Player. "He did it against Hull as well when he stepped in for Olivier Giroud, and this time you saw how vital it is to have the depth in the squad.
"It's important to have a second proper striker who comes in and just does what he usually does.
"Normally when you have a lot of chances at the end you struggle a bit and maybe you concede a goal, but at the moment we are quite solid at the back and don't concede a lot."
Mertesacker missed two excellent chances from set pieces in the second half and was relieved that they didn't prove costly.
“When there is a tight game you always get opportunities from set pieces," he said. "It was a tight game, it was 0-0 and you always try to get an opportunity as a defender when there is a set piece.
"I tried hard today and there were two good opportunities when I should have scored. But everything went well luckily."Copyright 2017 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