Morelos recently returned from a long-term absence due to a thigh injury and has come off the bench in the last two games.
Defender Filip Helander remains out with a foot problem and midfielder Ianis Hagi has been sidelined since January after suffering cruciate ligament damage
Former Watford midfielder Daniel Phillips is in contention to make his Saints debut after joining on a two-year deal this week.
Captain Liam Gordon, who is yet to play a first-team game this term due to injury, is fully fit and ready for action after coming through a B team game in midweek.
Murray Davidson is still suspended, while long-term absentees David Wotherspoon (knee), Chris Kane (knee), Callum Booth (Achilles) and Tony Gallacher (leg) remain out.
Probable Rangers team: McLaughlin; Tavernier, Goldson, Davies, Yilmaz; Lawrence, Davis, Kamara; Kent, Colak, Tillman.
Probable St Johnstone team: Matthews; McGowan, Mitchell, Considine; Wright, Hallberg, Phillips, MacPherson, Montgomery; Bair, Murphy.