Grygera makes no positive impact on the team. He either makes no impact or a negative impact. People think he played well because he usually doesn't make any obvious mistakes.
A guy like Melo will destroy an opposition's entire midfield but make two bad passes and get killed for it. Meanwhile, Grygera never even puts himself in a situation to make an impact.
There are simple plays that a much better fullback could make that Grygera doesn't but because it's not obvious or glaring no one cares. Simple things like getting forward more or supporting his winger or stopping a clearly interceptable pass. Grygera will just do nothing but hold his position because he's so limited. Or he'll let his man get in a cross so easily. Or he won't help out his CBs but because it's not REALLY his fault he won't get any blame when any good fullback could potentially help Chiellini or Bonucci.
But we've all come to know that play as "solid". Basically, he's like that guy at the office that does the bare minimum and no one complains.