I agree, I don't think his secondary pitch needs to be great by any means. Just something to give hitters extra hesitation that they can't always just sit on the heat like Goldy did on the 0-2 pitch. Estevez throws anything else there near the zone it's a K as much as Goldy got started early for the fastball.

Chapman is a good example, guy's slider doesn't break a ton but the change of speed and movement is enough to offset the gas and give hitters one more pitch to be aware of. Forces hitters to be defensive a bit more than they would be with only one pitch to consider.