A disjointed Hoops side had no answer to the Highlanders’ spirited defensive display as a first-half penalty from Ross Stewart and a header from Alex Lacovitti from a corner late in the second half sealed a famous win for Stuart Kettlewell’s men.
The home side's defeat – Celtic’s first in a domestic cup competition for 35 matches – extends a poor recent run of form for the Parkhead side both at home and in Europe and leaves the position of manager Neil Lennon hanging by a thread.
Here's how the Celtic players rated in an abject performance.
Player ratings scale - 10: One of best performances you've seen. 9: Outstanding. 8: Very good. 7: Good. 6: Average. 5: Poor, 4: Very poor. 3: Terrible day at office. 2: Abysmal. 1: One of worst performances you've seen.
9. Tom Rogic - 5
Started the game with purpose before fading out of the picture. Glorious flick to put Ajeti clear was the highlight of an otherwise poor performance. Photo: Alan Harvey - SNS Group
10. Odsonne Edouard - 5
Another quiet game for a striker capable of ripping Scottish Premiership defences to shreds on his day. Still doesn't appear to be fully engaged. Photo: Craig Foy - SNS Group
11. Albian Ajeti - 4
Blew a huge chance to draw level when clear on goal but struck the post. Otherwise barely involved before being subbed. Photo: Alan Harvey - SNS Group
12. Callum McGregor - 5
Given a rest after hectic spell for club and country. On for the final 20 minutes but struggled to get himself into the game. Photo: Craig Foy - SNS Group