The 26-year-old right-handed pitcher was taken by the Phillies from Boston in the Rule 5 draft during the league's winter meetings in December with hopes he would play after completing his military service.
If Song clears waivers, Boston will have the option of taking him back to its organization.
Song reported to major league spring training on Feb. 23 after the Navy granted his transfer from active duty to the reserves. Song hadn't pitched in a professional game since Aug. 29, 2019, for Class A Lowell.
Song went 1-0 with a 7.36 ERA in eight games in stints with three of Philadelphia's minor league teams while on the major league injured list with a lower-back strain. Song's 30-day rehab assignment ended this week, forcing the Phillies to decide whether to add him to the active roster or expose him to waivers and offer him back to the Red Sox if he is not claimed.
The Phillies are in the thick of the NL wild-card race and opted not to use a roster spot on a pitcher who had not pitched in any baseball game before this season since 2019.
"(The major-league coaching staff) hasn't really seen him," Phillies manager Rob Thomson said of Song before his team played the Pirates on Saturday night. "We trust the people who have seen him and if they think it can work then we try to make it work. There just wasn't enough there."
The 6-foot-4, 200-pound Song was picked from the Red Sox in the draft for unprotected minor league players. Philadelphia put him on the military list while he continued active duty. The Red Sox drafted Song in the fourth round -- he likely would've gone much higher, but his impending military service caused teams to back off.
Song impressed in his only pro season, making seven starts for Boston's Class A Lowell affiliate in 2019, with a 1.06 ERA and 19 strikeouts in 17 innings. With a fastball clocked in the upper 90s mph, he dominated that year as a senior at the U.S. Naval Academy, going 11-1 with a 1.44 ERA and 161 strikeouts in 94 innings.
In November 2019, Defense Secretary Mark Esper signed a memo clearing the way for athletes at military academies to delay service commitments and play pro sports after graduation. Song's request to have those rules retroactively applied to him was denied.
Song began school as a flight officer in the summer of 2020 and finished that phase last April. He started additional aviation training the next month.