With just two points between the top two in the Scottish Premiership title race, there’s no margin for error.
Last weekend, Celtic nearly slipped up at home to Motherwell; Josip Juranović's bizarre own goal gifting the visitors an equaliser.
Nevertheless, Reo Hatate’s rocket, followed by Callum McGregor’s cynical foul, that resulted in the captain being sent off, saw the Hoops to victory.
Ange Postecoglou’s team may be top of the table, but they now find themselves bottom of their Champions League group.
On Wednesday evening, the Celts paid the price for missed chances and slack defending, as usual in Europe, in a 3-1 defeat to RB Leipzig.
Jota did score a stupendous equaliser, but Joe Hart’s poor pass gifted the Bundesliga side a second go-ahead goal.
Celtic’s next two Champions League games see Leipzig and Shakhtar visit Glasgow, needing to win both to have a realistic chance of knockout football.
Before Tuesday’s huge game, will the Hoops avoid dropping points in Perth?
As for St Johnstone, they’ve enjoyed a mixed week that began with a Tayside victory over Dundee United last Saturday.
Stevie May and Melker Hallberg on target, much to the delight of a big traveling away contingent at Tannadice.
However, Callum Davidson’s side could not back this up on Wednesday, beaten 2-1 by Kilmarnock at Rugby Park.
This leaves the Saints ninth, five clear of the bottom two.
Will Celtic be far too strong at McDiarmid Park?