Quincy Jones chose her, not Michael Jackson.
Siedah Garrett was one of Quincy Jones' staff songwriters at the time.
Quincy Jones asked her if she liked the song and if she could sing it, and she replied in the affirmative.
Then, he sent her to the studio to record her vocals on the song.
Quincy Jones also ordered Michael Jackson to follow her so as to record his vocals on the song, as well.
"Sid, do you like this song? … You think you can sing it? … Well, get up and go in there and sing the song … Michael, go in there" (Quincy Jones)
His decision to abandon the Jackson 5/The Jacksons in order to begin a career as a solo artist can be considered smart.