Martin O'Neill admitted Celtic cannot afford to lose Sunday’s top-of-the-table showdown with Hearts at Tynecastle.
The Jambos are currently six points clear of both the Hoops and Rangers in the race to win the William Hill Premiership.
Defeat in Edinburgh would leave O’Neill’s side facing a nine-point deficit in their quest to retain their title.
"It’s going to be very big, no question about it," O'Neill said on Friday. "If we get beaten, it’s a long way back for us. It is a bit of a trek at the minute, so we would go out to try and win the game if we can.
"It’s not the end of the road (if Hearts win) but nine points would be a lot to claw back with 15 games left."
Celtic have lost both of their previous matches against Hearts this term – under Brendan Rodgers and then Wilfried Nancy – but O’Neill dismissed any notion that his side, who drew 2-2 away to Bologna in the Europa League on Thursday, might be happy with a point.
"I don’t think that’s in their nature," he said. "If you look at last night, the way we started the game, we closed the players down, we scored a goal from closing players down.
"So we went to take the game to Bologna. What’s the point in sitting back? Obviously, as the game goes on, we get a man sent off and second half, we had to retreat.
"They drove us back, as you would expect. But for us, we will be as positive as we possibly can, and we’ll try and win. If we can’t win it, we’ll try and save it."
O’Neill revealed that striker Johnny Kenny is a doubt for Sunday after picking up a calf strain during his substitute outing against Bologna.
"I think he’s gone for some sort of scans this afternoon, but we’ll see," said O’Neill. "He might be a doubt, I’m not sure. He just said he felt it during the course of the time he was on. Hopefully other than that, we’re ready to go."
The absence of Kenny would be offset by the arrival of new Czech striker Tomas Cvancara, who joined this week on loan from Borussia Monchengladbach and the possible return to contention of Kelechi Iheanacho after injury.
Cvancara has not played since featuring for Turkish club Antalyaspor during a loan stint on 24 November, but insisted he is psyched for making his debut in Sunday's high-stakes trip to the table-topping Jambos.
"Yeah, I had preparation in Germany, basically full January, so for me it’s no problem to jump in," the forward said on Friday. "So I would say that I’m ready.
"I trust myself, I trust the team, I trust my qualities and I’m not afraid to step in, so if the manager needs me, I’m there."
