Nady is dynamite vs. lefties, has not shown he can hit righties. He was worth owning this opening week, but might not be so hot this week as the Friars will face mostly righties. Roberts is then slated to return, which means Nady goes into a strict platoon in all likelihood. Thus, his only likely long term value is on a deep league roster where you can afford to bench him whenever SD is not facing a lefty starter, which will be most of the time.
This does not kill his value in these leagues, though. In one of my leagues last season, I maintained a 6 OF rotation all season and got a nice reward with Eli Marrero who tore it up vs. southpaws all season.
But if you're considering Nady as an everyday OF, it's not a good move.