Ranked as the Marlins' No. 4 prospect by MLBPipeline.com, Peters turned 25 on Thursday, and a day later he was a September callup.
"It was pretty awesome, from the first pitch to the last pitch, I'd say it was everything I expected," Peters said. "It's just a blessing to be out there and accomplish a dream."
Peters had a strong support group at Marlins Park, with roughly 30 family members and friends in the crowd, including his mother and father.
"That was awesome," Peters said. "My mom doesn't make it to a whole lot of games. When she does, for some reason, I pitch well. My dad, a bunch of relatives, friends, high school friends, college friends. They were all here. It's pretty awesome. It's cool to see a group of people here who supported you from the time you started playing baseball until now."
Peters breezed through seven innings on 91 pitches, with 54 strikes. He was at 77 pitches through six innings, came out for the seventh and struck out the side.
"Well we didn't know him, so that makes it a little more difficult," Phillies manager Pete Mackanin said. "He pitched well. He's got a real good curveball. I was pretty impressed by that kid."
According to Baseball Savant, Peters threw 38 four-seam fastballs, maxing at 93.5 mph. The lefty spun 36 curveballs, and got seven swinging strikes on the pitch.
"He was pitching, and pitching with command, and aggressive," Mattingly said. "A lot of what we were able to see in the spring as far as composure, handling himself out there. He did a nice job in lots of areas."
His final pitch was a curveball that fanned pinch-hitter Ty Kelly.
"I just knew I was throwing it as hard as I could," Peters said. "It needed to be down. It was tight enough tonight, and I was getting swings and misses on it. It just needed to be down. I couldn't miss over the plate."