Following their 2-1 Champions League defeat by Benfica in Lisbon in midweek Hoops boss Neil Lennon had claimed fatigue was impacting on his squad and it looked that way as they never got going against the neat and tidy visitors.
Kris Commons and Mikael Lustig missed good chances for the home side in an otherwise dull first half but there was no second-half surge from the champions and indeed, after McKay notched in the 65th minute from close range, he later spurned a good chance to double their lead when he hit the bar with only Fraser Forster to beat.
It was an afternoon of frustration Lennon who made three changes to his starting eleven. Commons, Beram Kayal and Tony Watt were drafted in with Kelvin Wilson (flu), Joe Ledley (groin) and Scott Brown dropping out - the Hoops skipper shrugging off an on-going hip complaint which may require surgery to take his place on the bench
Inverness boss Terry Butcher made one change to the side which lost 5-1 at home to Motherwell last week, with Philip Roberts in for Gavin Morrison.
Midway through the first half, Celtic defender Charlie Mulgrew picked out Hooper with a cross which allowed the striker to set up Kayal at the edge of he box but the midfielder's shot was blocked by Caley skipper Richie Foran and the ball spun through to Caley goalkeeper Antonio Reguero.
Moments later at the other end, Owain Tudur Jones set up Andrew Shinnie but his 20-yard drive limped wide of the target before the champions then squandered two good chances to break the deadlock.
In the 39th minute, as the Parkhead side broke swiftly, Hooper sent Commons through but the Scotland midfielder's angled-shot from 12 yards was wayward. Two minutes later Lustig had a similar and possibly better opportunity when Hooper cleverly allowed a Kayal pass to run through to the right-back but he hit the side-netting with his unconvincing effort.
Celtic began the second half as they ended the first, on the front foot, but with still several gears to find but Caley's goal was not a surprise and came through another swift counter.
Roberts took a pass from Aaron Doran inside the Celtic box, turned inside Adam Matthews and Mulgrew, and fired the ball across for McKay to guide the ball past Forster, with the home fans claiming offside.
Source: PA
Source: PA