Welbeck struck his treble in their Champions League rout of Galatasaray.
"He is the main threat at the moment, especially on the counter-attack," said Mertesacker.
"We have players who can put him in the right situations and that is what we need to find ourselves.
"It took a bit of time, but I think now we are on (the) spot and we see what it takes to be a team and to be fully integrated."
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