In a word, the man is consistent. Look at his averages over 34 games here:
That is his stat line practically every week.
in 2017 his numbers are:
Pretty much identical to what he has done over his entire 34 game span here. He is what he is, which is also a GOOD THING for trying to win games. McDermott knows what he is going to get and can plan around it. He knows he'll get 1 or 2 TD and 200 yards with a turnover every other week.
Is it good enough to really win in this league? I don't think so. People point to the Chiefs and Alex Smith as a model. As good as the Chiefs are we all know what is going to happen when they play NE when the game really counts. Their career numbers are pretty close though (Smith and Taylor) although this season Smith is averaging about 100 yards a game more than Taylor, which is a career anomaly.
So yea, it can get a LOT worse. If a QB like Tyrod (who doesn't put up the passing yards to keep you in games or catch up) turns it over more you're likely 1-4 or 0-5 right now.
A guy like Tyrod has value as a solid backup long-term, but he is also a guy who has had just enough success to expect a starting role. It's too bad because he would be an amazing backup QB for the long-term guy. He's literally the definition of the perfect backup: consistent, takes care of the ball, good teammate, can get you a few wins without losing you games.