The England forward returned from a nine-month spell on the sidelines to score an injury-time winner against Leicester City on Sunday.
Arsenal are now just two points behind the league leaders and Mertesacker believes that Welbeck’s impact will give the Gunners a boost going into the final months of the season.
For Danny to go through a bad, injured period where he kept believing in himself, when everyone maybe doubted him, it was a great chance for him to come in and show he was ready to give everything for the teamPer Mertesacker
“We need players coming back,” Mertesacker said. “We have long-term injuries, which gave us a bit of a struggle, especially in January, where we played in December and January with almost the same team.
“The players gave us great energy and stepped in for other players but now we need players to come back and make the difference. That’s very important, as you could see against Leicester.
“We are happy to have Danny now and a few others are not far away. We need our players - you could see that that makes a difference. We’re happy to have at least him back and there’s more coming back as well.
“For Danny to go through a bad, injured period where he kept believing in himself, when everyone maybe doubted him, it was a great chance for him to come in and show he was ready to give everything for the team. He was always integrated into the team but when you feature on the pitch, give the team such a great goal and energy, it gives him a big lift.
“Also, that winner at the end gives you something. We hope that we stay focused on ourselves because how we defended in the first half was good. We limited them, [prevented them] from countering and that was the important thing. We kept believing that we could score against them.”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