The Great Britain international has not yet recovered from ankle injury sustained in a home defeat by Bradford on August 19.
?It has been a bit slower in healing than we hoped,? admitted Leeds coach Tony Smith, preparing for Friday?s match.
?We are being very conservative and cautious with it. It is improving, but not to the degree we would have liked - so he won?t be available for this week.?
Leeds are hoping Senior will be fit for the Grand Final on October 15 if they make it to Old Trafford for a second successive season.
St Helens 18-man: Wellens, Gardner, Lyon, Talau, Hooper, Fozzard, Cunningham, Gilmour, Wilkin, Higham, Bennett, Mason, P. Anderson, Graham, Hardman, Roby, Fa'asavalu, V. Anderson.
Leeds 18-man: Mathers, Calderwood, Walker, Bai, McGuire, Burrow, Bailey, Diskin, McDermott, Lauitiiti, McKenna, Sinfield, Dunemann, Ward, Poching, Jones-Buchanan, Ellis, Smith.Like