The Celtic manager, Lou Macari, said McStay had a groin injury and there was a danger he could have a hernia which would have to be treated either now or at the end of the season.
Macari is also without Rudi Vata with a broken arm, Gary Gillespie (achilles), Tom Boyd with an ankle injury and Brian O'Neil. Tony Mowbray is still suspended. With so many players out, Macari will be forced to field a scratch defence of Barry Smith, Mike Galloway, Mark McNally and Lee Martin.
Defeat for Dundee would make them the first club relegated from the Premier Division this season.
With today's game against Dundee United followed by Motherwell in midweek and Celtic at Ibrox next Saturday, Rangers begin a week in which three wins could see them seal their sixth title in a row. They are unbeaten since United beat them 3-0 at Ibrox in December.
United have a doubt over Billy McKinlay, who scored for Scotland in Austria in midweek, and will lack Craig Brewster, who is suspended.
The race to avoid relegation speeds up with four clubs - Partick Thistle, Hearts, St Johnstone and Kilmarnock - in the fight. Hearts make the trip to McDiarmid Park to play St Johnstone without a win at the ground since
December 1991. Alan McLaren is still suspended, Jim Weir will take his place, while Scott Leitch is injured.
St Johnstone are without the suspended Harry Curran and Ian Ferguson. Partick Thistle travel to Aberdeen with Grant Tierney free after suspension. Kilmarnock will be without the defender, Craig Paterson, against Raith Rovers.Reuse content