I have no idea what's wrong with him, but to blame it on lacking mental toughness is strange, among many other things.
It's the easiest way for fans to explain a player's struggles, and for whatever reason, it's more often used as a rationale in baseball than any other sport.
I'm a Mississippi State fan, and it's the same thing with MSU baseball...if a highly touted player struggles for a stretch of time, even if young and inexperienced, then they 'lack mental fortitude'. If another, less highly touted player outperforms their expected production, even if for a short time, they are mentally tough. It's pretty dumb, really.
Teheran is struggling on the road badly; that is obvious. We can't trade him now when his value is at its lowest. He is almost sure to come out of this and pitch much better, and when he does, we can then have a discussion about whether he should be moved.