They would start, presumably, with the Dens Park side when they host the champions tomorrow afternoon.
It is the first match which Celtic play after being knocked out of the Champions League before the group stages. They learned their group opponents in the Europa League yesterday but that is unlikely to have salved the wounds inflicted on them by Maribor earlier this week.
Hartley is acutely aware Celtic will still be smarting tomrrow and has warned his players to expect a backlash. "We know they'll be up for the game on Sunday. They've had a lot of criticism," said the Dundee manager, who is without both Willie Dyer and Simon Ferry at Dens Park.
"We know we're in for a tough 90 minutes. I think their players will be hurting. They will want to show that they're still a good team and I do definitely expect a backlash. Celtic have always been like that."
Kyle Letheren, the Dundee goalkeeper, did not appear to be fazed yesterday, though. "All the pressure is on Celtic as they have had a few bad results. We have nothing to fear, really," said the Welshman.