Certified GKH Gardening Expert
You don't need to create a whole new bed, but it is best to at least remove the grass out to the dripline and add 2 to 3 inches of mulch. This will help keep the mower and/or weedeater from hitting the trunk and help retain moisture.
Also, if the plant is completely surrounded by grass, it will have to compete for nutrients and water. And when you apply any lawn fertilizer or herbicide, you will need to make sure it is applied several feet from the shrub to avoid its root zone.