He does have a great arm He also has two nasty MLB-worthy pitches (sinker-slider).
His problems:
1) All his secondary pitches are subpar. That makes him heavily dependent on those two pitches.
2) One of those two pitches "off" that day...he becomes very hittable.
3) His control isn't great. Bad Repeatability? When his control is off he's walking lots of batters.
4) The above reasons are why he's had a 1.50-ish WHIP his entire career. Gives up too many runs on a game-by-game basis.
But...
When he's on top of his control over his two nasty pitches... and can toss a 4-seamer or changeup or curveball just to keep hitters off balance... he can pitch (starter) a nice little gem. Those are... too far and in-between for me to like him as a starter though.
He either needs to improve control/ improve secondary pitches (to remain a starter, IMO.)
Or just move to the bullpen. I think he becomes much more effective in there, where he can show off his "great" arm to max effect.
With too many injuries right now though, he's stuck as a starter.
Get Melton & Jobe back later this year and I SO want to move Montero over to the bullpen.
Just my 2 cents.