Rangers leapfrogged Celtic to take top spot in the Scottish Premiership with a 2-0 win over Ross County on Sunday ahead of the first Old Firm derby for 10 months after the international break.
Celtic were also 2-0 winners at St. Johnstone, but left it late as Leigh Griffiths and Patryk Klimala came off the bench to score in the dying minutes.
Neil Lennon's men trail Rangers by a point, but have a game in hand and will have home advantage for the first Glasgow derby behind closed doors on October 17.
Both sides secured their place in the group stages of the Europa League on Thursday, but suffered a European hangover.
Rangers went in front early at Ibrox through James Tavernier's penalty, but struggled to build on their lead.
Coll Donaldson wasted a big chance to level for County in the second-half before Brandon Barker secured victory for Steven Gerrard's men two minutes from time.
Celtic left it even later as they had just one shot on target until the 90th minute in Perth.
Lennon emptied his bench in the search for a winner, making five substitutions, and one of them made the breakthrough.
Griffiths was introduced for his first appearance of the season after being publicly criticised by his manager for returning to pre-season overweight.
However, the Scotland striker showed he maintains his predatory instincts in the box as he headed in Hatem El-Hamed's cross as the game entered stoppage time.
"Delighted to be back involved," said Griffiths. "For me personally it's about getting fit.
"The game is 90 minutes long, we know we are going to have chances. It didn't come till the 90th minute but two goals again, a clean sheet and we move on."
Moments later, Klimala's persistence was rewarded with his third goal of the season as he bounced back to his feet after being wiped out by Liam Gordon to slot high past Zander Clark.
"The squad wins today in terms of the depth we've used," said Lennon. "The first-half we didn't work the goalkeeper at all and that's not what we are about. The second-half we were much better."