Concentration is pretty much a forced type of activity. It can be developed by force, by sheer unremitting willpower. And once developed, it retains some of that forced flavor. Mindfulness, on the other hand, is a delicate function leading to refined sensibilities. These two are partners in the job of meditation.
Mindfulness is the sensitive one.
He notices things. Concentration provides the power. He keeps the attention pinned down to one item. Ideally, mindfulness is in this relationship. Mindfulness picks the objects of attention, and notices when the attention has gone astray.