I do think Rodon will actually be pretty successful this year. He's mccullersing which is good, and his slider is varied so it's not so simple as to classify it as a single pitch.
But when his fastball is as bad as its been, and when he apparently does not bother with either a 2seam/sinker or changeup, he has a ceiling that I can't imagine more than a decent 3 starter.
For real though, when he needs to he can just go slider every pitch in an at bat and it's very unlikely they will get a hit.