Martin O'Neill says it would be "a long way back" into the title race if his Celtic side lose at Tynecastle on Sunday.
A third defeat of the season to Derek McInnes' Premiership leaders would leave Celtic nine points adrift with 15 points left.
"It's going to be very big, no question about it," said interim manager O'Neill.
"If we get beaten, it's a long way back for us and it is a bit of a trek at the minute. So we will go out to try and win the game if we can.
"I think that it's not the end of the road [with a loss], but nine points would be a lot to claw back in so many games left."
Celtic head into Sunday's game after playing for almost an hour with 10 men in the 2-2 Europa League draw with Bologna in Italy on Thursday night.
When asked if the players would be happy to play for a point on Sunday, O'Neill added: "I don't think that's in their nature.
"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, second half, we had to retreat.
"They drove us back, as you would expect, they're very, very fine side. It was all hands to the pump. But that's the way the game goes.
"We will be as positive as we possibly can and we'll try and win. And if we can't win it, we'll try and save it."
