The difference between a marginal #5 and a solid #2 is often one pitch.
Even Nola is a much better pitcher when he has his changeup working.
Velasquez has a 4 seamer with great movement, but has never really had reliable secondary pitches.
His curveball and changeup got pounded in 2017 and 2018, which is why they emphasized the slider, which was an out pitch in 2018 but got pounded in 2019, while his curve was an average pitch.
To be a starter, they don't have to be great, but they have to be at least average.
To be a RP, he just has to be above average with one of them.
Pivetta has good velocity (95+) but no movement and his 4 seamer gets hit like a Pinata.
He's got a plus plus curve at times, and an average slider.
Teach Velasquez to sharpen his secondary pitches and he makes a big jump.
Teach Pivetta to grip his 4 seamer differently to generate movement and he makes a big jump.
Add either an effective 2 seamer or changeup and he takes another step up.
Suarez has a solid 2 seamer and 4 seamer, but his changeup and especially his slider need work.
Think the key for Eflin is to improve all his pitches incrementally, 4 seamer, 2 seamer, slider and changeup, especially the changeup, which incidentally is the specialty of the new PC.