I'm not smart enough to evaluate an offensive coordinator. I'm not going to sit here and pretend to have the knowledge that says Garrett is good, or Garrett is bad. I don't have it.
What I do know is... when I played football.... when we went out there and executed the game plan, the coach looked smart. When we went out there and laid an egg, the coach looked like an idiot.
I played OL. What I learned from it, was, when we gave our QB time, he did well. And when we got abused (and there were games that it happened), the offense was out of sorts all game long... and the coaches looked stupid (according to our parents/fans/fellow students/friends).
Yeah, the coaches would try to put us in position to succeed, but ultimately, it hinged upon how we went out and executed it.
No idea if Garrett is a good offensive coordinator or not.
No idea if he will make a good head coach or not.
But I do know, if the players execute poorly, nothing will happen. Regardless of who is calling the plays.