Kyle Lafferty's pin-point finish condemned Celtic to a 1-0 Ladbrokes Premiership defeat at Hearts.
The former Rangers striker met Uche Ikpeazu's ball in from the right first time on the volley and his low shot from just outside the area arrowed into the bottom corner.
The goal after 57 minutes made it two wins from two for Hearts at the start of the new season.
In contrast, Brendan Rodgers' visitors, who were held to a 1-1 draw with AEK Athens in their Champions League qualifier on Wednesday, struggled to impose themselves against the team who ended their 69-game unbeaten domestic run last season.
The Northern Irishman would not make any excuses for the Jambos defeat - sealed by Kyle Lafferty's goal - saying: "Three game in seven days is very tough but not an excuse. We have to be able to cope with that. One mistake cost us".
Hearts manager Craig Levein admits he smelled blood when he saw five changes to Celtic's side amid two Champions League qualifying games.
He said: "I don't know what Brendan thought but he had serious decisions to make, the Champions League is worth a helluva lot more money to him than beating Hearts away from home.
"I am absolutely positive he could do both but today he did make changes and it was another thing, along with our form, along with recent performances here against Celtic; positive thoughts for us to go into the game absolutely believing we had a chance of winning."
Levein is concerned about skipper Christophe Berra who was carried off the pitch before the end of the first half with a hamstring injury.
He said: "It is a hamstring injury. How bad it is? It doesn't look great. He is still struggling to walk. That is a concern for me. It is a big loss."