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.
5. Kristoffer Ajer - 4
Looked uncomfortable on the left side of the back three. Slow to react to danger at times, and poor decision-making with his passes. Expect better from a player with a supposed £20m price-tag. Photo: Craig Foy - SNS Group
6. Diego Laxalt - 5
Got into one or two promising positions but failed to provide the quality from wide areas that Celtic required. We're used to seeing a far higher standard from the AC Milan loanee. Photo: Rob Casey - SNS Group
7. Scott Brown - 6
The scapegoat for the midweek defeat in Prague did all that could be asked of him in this game. Broke up play and got the ball to the more creative players, who failed to deliver. Photo: Alan Harvey - SNS Group
8. Ryan Christie - 5
Had one skilful moment in first half that drew a free-kick on the edge of the box, but otherwise struggled to produce the magic that Celtic have often relied upon. Deliveries into the box were poor. Photo: Craig Foy - SNS Group