The Phillies signed Robertson to a two-year, $23-million contract this offseason with a club option for 2021.
Based on his recovery timeline, Robertson will miss the bulk of the deal after completing 6 2/3 innings before being placed on the injured list.
The 34-year-old has generally been a reliable reliever, having pitched at least 61 innings in nine straight seasons entering 2019. He owns a lifetime ERA of 2.90 and has recorded 137 career saves.
In his absence, Hector Neris has served as the Phillies' primary closer, converting 21 saves with a 3.38 ERA and 12.26 K/9 in 50 2/3 innings.