DeSclafani will make his major league debut tonight against the Dodgers, making the jump straight from Double-A. Part of the reason he was selected to make the start was that his rest lined up with the Marlins need for a starter.
As it would be for anyone, the drop off from Fernandez to DeSclafani will be large, though there are questions about whether DeSclafani will be able to remain in a big league rotation long-term. He profiles as a fourth or fifth starter, with a fastball that sits between 90-94 and an above-average slider that works against righties but isn't a weapon against left-handed hitters. His change-up is a work-in-progress and the whole arsenal plays up because of plus command. DeSclafani's best weapon is his ability to throw strikes. His 2.1 BB/9 rate this season is not only quite good, but represents a career worst.
The lack of an quality off-speed pitch will hold him back against left-handed hitters and as he tries to circumvent a lineup for a third time in a game. His fastball/slider combo, however, could play well in a bullpen situation.
His role with the Marlins likely has more to do with their needs than his ability. As more pitching prospects arrive in the majors, DeSclafani is a likely candidate to get bumped to the pen. In the meantime, however, he can help keep them afloat and represents good organizational depth. There are a lot of organizations that have had to reach down into their farm systems for pitching help this season and are pulling out pitchers much worse than DeSclafani.