Two good pitches aren't enough to be a starter. He has no third pitch whatsoever.
In 19 career starts he's pitched 89.1 innings and given up 50 earned runs with 68 Ks, good for an average of 4 2/3 innings per start with an ERA of 5.60 and K/9 of 7.6. His 2018 numbers as a starter are even worse (5 GS, 18 IP, 7.22 ERA, 7.2 K/9).
But last year in relief, he tossed 26.2 innings and gave up 13 earned runs with 26 Ks. While the ERA isn't great at 4.86, it's certainly better than his numbers as a starter. And as his improved K rate shows, his stuff plays up in relief since he only has to go through a lineup once.