I know I'll get voted down for this, but as we saw in the Bills game, the roster he built was probably good enough... as long as nothing went wrong. Like everything had to go just right for this team to succeed.
The injuries at Oline and then to key parts of the defense just did us in. Mac can't concentrate on his fundamentals when he's convinced he's going to get creamed on every play. The WRs don't get enough separation for Mac to get them the ball with his mistimed and out of sync throws.
And yeah, he should have built the team better, with more depth. Most of that is on too many draft picks that just didn't work out. But I doubt you'll get him to admit that. In his mind, the team he built can win. As long as nothing goes wrong.