Right after QB, TE is the hardest position to play on the offense because the mental part is so critical. Witten's knowledge of defense and how they line up is his greatest strength. And he was a quick study for Parcells to trust him as much as he did. When the D is bringing it, it is the TE's responsibility to bail his QB out and how many times did we see Witten do that for Romo and then Prescott in his rookie year? Countless, that many.
Gronkowski has become a very good blocker and I can recall him addressing Witten's completeness to his game as the motivation for that. And that was harder for him than running routes and getting his bigself open. But he worked on it.
There are players with intangibles that we do not see, we only hear about them. No player exhibited "team" better than Witten when his best friend was benched for a rookie and then not allowed a shot back at his job. You know every coach and player on that team looked at him to see how he would handle it. He's not a vocal guy but he still sends messages. Like that time he excused himself from his interview and went over just to stand by Dez as he was going off on a reporter and asking Dalrymple for help. Think any other player on the team could have done that? Not the first time he tried to get Dez in off the ledge.
When a team has that certain intangible, that becomes more apparent when it's missing. Garrett saw that and so did the other coaches, part of the soul of the team was missing, the mental toughness. Witten is not just back to block, run routes, read defenses, catch passes and make 1st downs. Jason Witten is back to be Jason Witten.