Butler, who was the team's seventh-round pick in June out of the University of North Carolina-Charlotte, is off to a 3-1 start with a 0.92 ERA in his first five starts with High-A Lake Elsinore.
He's rated as the No. 22 prospect in the Padres' system by MLB.com.
Butler's fastball sits in the mid-90s, but he isn't just a power arm, said first-year pitching coach Glendon Rusch.
"He's not a power arm you have to worry about not being able to throw the ball over for the plate," Rusch said.
Butler, who relies on a fastball, slider and changeup, has struck out 20 and walked five so far.
He moved fast last summer after he signed, starting in short-season Eugene before moving to low-A Fort Wayne, where he closed games, posting an 0.83 ERA in 18 games with 10 saves.
"I think the decision to have him start was to really let him refine his pitches," Rusch said. "… You can always become a reliever later, I think."
Rusch praised Butler's mound presence and how he "doesn't get rattled."
Butler threw 97 mph on his 95th and final pitch on April 30 in a start against Inland Empire.
"He's really maintained his velocity throughout all of his outings," Rusch said.
Butler, who stands at 6-foot-6, was picked in the 13th round of the 2013 Draft by the Yankees, but he chose not to sign. His fastball touched 97 mph in a pre-draft workout for New York.
UNC-Charlotte was Butler's third school after attending Marshall, and later Northwest Florida State.