The England international was often used as a winger during his time at Old Trafford, and he feels that he was forced into the decision due to his desire to prove his worth in a central role.
"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," Welbeck told members of the media.
"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."
Welbeck was a starter as Arsenal recorded a 2-1 win over Anderlecht on Wednesday.