Does he need an origin: NO
Why not: Arrogant surgeon Stephen Strange loses the technical use of his hands after an accident, thus causing him to seek out a cure that ultimately leads to him getting magic powers and becoming the Sorcerer Supreme! It's not the worst story ever told, but, as I mentioned earlier, you can cover it in an opening credits sequence and move on.
Plus, Strange is an older guy. Who wants to see someone in their forties hem and haw over their place in the universe? It's only barely tolerable when teenagers do it because they, at least, are young and dumb.
Bottom line: The Marvel movie universe doesn't need an introduction to Strange -- they need an introduction to how magic fits into the picture. It's better to have a seasoned Sorcerer Supreme as our protagonist to accomplish that goal.