Here is a good article on Rosenthal with an interview of his community college coach:

“When we first saw Trevor play, he had a live arm and was very athletic!” Burroughs told me. “He played Short Stop and pitched while here and we visited with Trevor and explained to him that his future in professional baseball was as a pitcher. He agreed and that is when he started to progress at a rapid pace. His fastball velocity started to increase and his composure was really good. We used him as a closer and the best velocity he showed here was in the Junior College World Series at 97mph. It was easy to tell he had a high ceiling. We are proud that he is a product of our program and hope he has continued success as a Cardinal!”
As you can see, he already had stuff but it was really really raw in college. No need to go down the PED bandwagon here unless you're going to accuse him of using PED's in community college, 757690