McCullers does look good, but it's a very small sample size. He was solid (not great) in the minors and has pitched 24 innings in the majors so far. He looks great, sure, but I think we need to wait before saying he is better than Alvarez, who has already pitched two very good ML seasons.
As far as McHugh goes, he turns 28 this month and has a lifetime 4.22 ERA and 3.74 FIP in 272.2 innings. He might turn into a decent back-end-of-the rotation starter, but are we even sure he is any more desirable than Cosart (who is three years younger)?