Let's take a look at why Franklin Morales is having success as a starter.
Franklin Morales has a Z-score of 1.0 or higher for whiff/swing on every pitch he threw in june. That means he can get people to whiff at all his pitches, fastball, sinker, curveball, changeup are all getting above average swing and misses. Also Morales' fastball has the best Z-score behind his sinker which is a fastball variant. Fastballs are the most common pitch, so to get lots of swing and misses on those is very important. So let's ask the question of why is Morales' fastball is so effective. Well first of all he has had an average of 94 MPH fastball as a starter, so his fastball will probably be good, but Morales' fastball also has a spin rate of 2,056 RPM on his four seam and 2,513 RPM on his two-seamer. Those are very good and trackman did a study that showed that pitches that spin more are much more effective. Morales' curveball is also fantastic movement wise. An average left handers curve has 6.13 inches of drop and 3.88 inches of horizontal movement away from a lefty. Morales has 7.46 inches of drop and 6.12 inches of horizontal movement away from a lefty. Morales' curve also has tremendous late movement. If you look at the graph below, his curve drops more than a foot from five feet away from the plate to where the batter hits it. This can be very difficult to hit.