McArthur was added to the Phillies' 40-man roster on Friday.
McArthur doesn't come with a ton of prospect pedigree as a 12th-round pick in the 2018 draft, but he's evidently shown enough that the Phillies were worried about losing him in the Rule 5 draft. In 15 starts and four relief appearances for Double-A Reading last season, he posted a 4.48 ERA, striking out 24.4 percent of opposing batters while walking 7.2 percent.