“Their one task is to look at what is happening in the six-yard box and on the goal line. You can’t blame the main referee for that. Referees make mistakes as well, but this mistake was very hard to take.
“It’s a hard blow for the boys and for us. We’re very disappointed and we had control 11 versus 11 and we should have killed the game off.”
Deila had no complaints about Craig Gordon’s dismissal.
He said: “It’s one big decision that has changed the game for us. Everyone will tell you the same.
“You would need to ask the referee and the assistant behind the goal why they thought it wasn’t a penalty.
“I asked the centre-half during the game if it hit his hand or his face and he said ‘hand, 100 per cent’.
“We have the best penalty taker in the country in Kris Commons so I have no doubt he would have tucked it away and they are down to 10 men and have a mountain to climb.”
While hopes of a domestic clean sweep are over, Deila is looking for the Premiership leaders to keep on course for the title against Dundee at Dens Park on Wednesday night.
The former Stromsgodset boss said: “It’s a tough world and that’s why it’s so hard to get a treble. It’s small details that can make you fail and today it was a small detail that made it hard for us.
“We have to get that behind us as quickly as possible and focus on Wednesday. Good teams bounce back quickly.
“Everybody is very disappointed today, but I’ve had disappointing moments before in football.