There is no tight end in the league right now as good as Rob Gronkowski, and there might not have been one before. His size, speed, catch radius and footwork separate him from his peers, and so does his ability to separate from defenders. Gronkowski is a perfect weapon for Tom Brady and his offense.
Because of the myriad ways the Patriots utilize Gronkowski, no one person will take on the task of guarding him alone. Sometimes it will be a linebacker. At other times it will be a cornerback. But a safety like Darian Stewart, with his size and speed, will surely see his fair share of defensive snaps defending the dynamic tight end.