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
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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.