Brendan Rodgers admitted his Celtic side lacked quality in their shock 1-0 Viaplay Cup defeat to Kilmarnock at Rugby Park.
Marley Watkins converted a Danny Armstrong pass in the 59th minute of a pulsating last-16 tie to send Derek McInnes’ side into the quarter-finals where they will host Hearts.
It was a first defeat for Rodgers in cup competitions in Scotland in 25 games and put the holders out.
The Celtic boss said: “We are very disappointed and it was certainly nowhere near the level that we need to be at to win the game.
“Kilmarnock showed their resilience and took their chance when it came.
“We were a bit more progressive when Kilmarnock got the goal and we started to be a threat.
“I thought Yang (Hyun-jun) coming into the game looked very well and he gave us that ability to take people on in a one-v-one but other than that we lacked quality in the game and it’s so disappointing.
“We were just not at the level in terms of performance.
“It is a tough place to come, of course, but we need to show more and we need to have a greater physicality in the game and then when you have won that battle you need to play with more quality and we didn’t play with that.”
Rodgers won seven out of seven trophies in his first spell as Celtic boss and insisted the defeat put that achievement into context.
He said: “I think when I came in here before and won trebles people think that’s just what you come in and do.
“It’s a very difficult thing to do and you see with this defeat that magnifies the achievement from before.
“It is a new cycle and I want to win every game as the Celtic manager and this is bitterly disappointing.”
Killie completed a remarkable early-season Old Firm double, having beaten Rangers 1-0 on the opening day of the cinch Premiership before a goalless draw against Hearts at Tynecastle.
McInnes was pleased with “such a strong start” to the season.
“I said to the players it looked an ominous start and one that would really test us,” McInnes said.
“But thankfully we have come through the three tests – Rangers, Hearts away, Celtic – with three clean sheets.
“That is testament to the work from the players. The thing that pleases me most at this early stage – because no team can be at their best at this moment – this was always the type of performance and type of team that I was looking for.
“Trying to get that identity and it feels like my team, this time around.
“Winning the league and getting to the Premiership was the first port of call, and we did it.
“Staying in the league last season was the objective and we did it, just. But this season the demand shifted and it was the first chance to put my squad together.
“We have signed a lot of players who weren’t playing at their last club.
“We are littered with those players and we want them to come here and see this is a good step for them, a good platform.
“And while they are here do something, try and create something special.
“Come up with big occasions and try and go as far as we can in the cup, and be a cup-winning team. It helps the optimism that is clearly about the place at the minute. But it is only a start.”