Welbeck joined Arsenal in a £16million deal this summer, despite interest from Tottenham and Hull.
And although the move only came together as the clocked down on transfer deadline day, the England international admits he had been considering his future at Old Trafford for the last 12 months.
Welbeck, 23, said: “When you’ve been somewhere for so long and then you’re going to start a new chapter in your career, you’ve got to think about it a lot.
“Over the past year or so, I started to think about what’s best for me because at the time I was playing on the left wing a lot and in a 4-4-2, which is very difficult for me because I can’t make an impact on the game.
“I can play it to the best of my ability but that’s not best for the team, and I could make a bigger impact playing in my preferred position (up front).”
Welbeck has scored five goals in seven games for his new club.