Derek McInnes blamed a "horrendous" refereeing decision as Kilmarnock suffered the loss of two late goals in the first leg of their Conference League play-off tie against FC Copenhagen.
The Ayrshire side produced a spirited performance before David Watson was deemed - after a VAR intervention - to have fouled former Celtic winger Mohamed Elyounoussi in the 75th minute.
Kevin Diks tucked the penalty away, before Rasmus Falk scored with the final kick of the game in stoppage time to fully punish the battling visitors.
"By and large we felt okay until the referee gets involved," McInnes told BBC Sport Scotland. "For me, it's a horrendous decision.
"It's extremely harsh. I don't think it's a clear and obvious error. It surprised everybody when he was asked to go over.
"It's a coming together and I don't think it's a penalty kick. It floored us a wee bit. I know how gutted I felt on the touchline.
"For all the effort, work and organisation - to lose a goal to that is extremely harsh."
Kilmarnock head into the second leg next Thursday at Rugby Park two goals down and with a mountain to climb to prolong their European adventure.
With the exception of last week’s heroics in Norway, Kilmarnock have struggled to reproduce their stoic performances from last season, but they looked reinvigorated on the biggest stage of their European journey so far.
For much of the first half, they reduced Copenhagen to potshots from distance with only one effort - from defender Denis Vavro - forcing the deputising Kieran O’Hara into a save.
Although Kilmarnock were holding their own, they didn’t create much either.
The best chance was a first-half flick on from Brad Lyons that Marley Watkins almost got on the end of, but Nathan Trott was able to tip the ball away.
The Copenhagen fans weren’t happy at the half-time whistle and the players came out of the blocks quickly, once more pinning their visitors back in the box.
They needed a bit of a helping hand for the opener, though.
As referee Fabio Maresca trotted across to the VAR screen, heads were scratched. What had been seen?
Replays showed Watson just about catching Elyounoussi, the penalty was awarded, and Diks slotted in from 12 yards.
It was a sickening blow for a team that had battled so far, but they’d be sent to the canvas with the final kick.
A corner was headed away by Innes Cameron, but Falk was able to shin the ball into the top corner as he reacted to the ball flying past him.
Exit beckons as McInnes' side flattened by late flurry
The low-block employed by Kilmarnock early on was hardly a surprise. What was more surprising was how well it worked, and how Copenhagen struggled to break it down.
Criticism has been levelled at the Kilmarnock defence this season - a far cry from last season when Lewis Mayo, Robbie Deas and Stuart Findlay all drew praise.
But despite the poor form of his back-line, McInnes' gameplan still relied upon it being up to scratch. Thankfully for him, it was back to its best.
Up top, Kyle Vassell lead from the front. There was almost no service to the big guy, yet he was able to hold up the ball when it came to him, unsure when he would see it again. He won corners and free-kicks, none of which were utilised to the full.
At the very least, Kilmarnock now have a free hit next week. There's no excuse not to go for it. Could we see Innes Cameron or Bobby Wales start?
What they said
Kilmarnock manager Derek McInnes told BBC Scotland: "The effort of the team and the shape - we gave ourselves every chance to get a positive result. They hadn't done much to trouble us and the game played out how I was hoping.
"We carried a threat, had some good moments in the first half, enough to give the players encouragement we could get something from the game.
"At half-time, the odds start to swing towards us a wee bit. We wanted to keep them at arm's length and make them do something out of this world to try and change it."
Kilmarnock defender Lewis Mayo told BBC Scotland: "It's hard not to get emotional and feel despondent. It's a really tough pill to swallow.
"We can't get too down about it, we've got another chance at Rugby Park. We need to stay level headed and calm."