No, it should go into the garden immediately. Full sun or as much sun as you can give it. Prepare a nice wide hole for it, not too deep with rich humus soil. Magnolias have fine shallow roots that grow just below the surface, not down too much. The top of root-ball should be even with the surface. Sprinkle some pine needles into the hole or as a mulch - this will acidify the soil perfectly. Keep soil constantly moist, a mulch would be a good idea.

Agree with Bathgate but don't let the mulch touch the trunk of your Magnolia, which one is it out of curiosity?

Good point, that's very important - not letting the mulch touch the bark. As far as pine needles go, I meant just a handful or two should do it. If you have a conifer growing nearby, you don't even need to bother. The naturally shedding needles will do it for you.