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In McWilliams, the D-backs get a 20-year-old who was drafted by in the eighth round by the Phillies in 2014 out of Beech High School in Hendersonville, Tenn., and has spent the past two seasons with Philadelphia's entry in the rookie Gulf Coast League.
"Sam follows along the lines of what we've been doing for the last year, acquiring young power arms," D-backs GM Dave Stewart said in a statement. "He's a high school draft pick that put up good numbers in the Gulf Coast League. He'll be a great addition to our organization."
MLB.com ranked McWilliams as the 27th-best prospect in the Phillies system.
McWilliams' best pitch is his fastball, which sits in the 92-95 mph range and has good sink. His second-best pitch is his changeup, followed by his curveball.
After compiling a 5.40 ERA in 2014, the 6-foot-7 McWilliams flashed more potential this past season with a 3.27 mark, striking out 21 and allowing five walks in 33 innings.
The D-backs acquired Hellickson last November from the Rays in exchange for a pair of Minor Leaguers -- Andrew Velazquez and Justin Williams.
Steve Gilbert is a reporter for MLB.com. Read his blog, Inside the D-backs, and follow him on Twitter @SteveGilbertMLB. This story was not subject to the approval of Major League Baseball or its clubs.