The Hoops are still smarting after exiting Europe on Thursday night following a 3-1 home defeat by FC Copenhagen, the Danish side progressing to the last 16 4-2 on aggregate.
Elyounouss passed up a great chance in the first half when he missed the target with a drive from 14 yards, but the on-loan Southampton forward insists the whole squad need to re-focus for their William Hill Scottish Cup quarter-final at McDiarmid Park and on their bid for a fourth successive domestic treble.
The 25-year-old Norway international said: "It is hurting a lot at the moment.
"This is obviously going to give us motivation for the league and the cup.
"We are going to fight our way back. Of course we wanted to be in the last 16, especially after what we did in the group stage and what teams we beat and how we played.
"That is unfortunate but now we can focus on the league and the cup.
"I am sure the boys are frustrated and disappointed but we will be back on it and give everything.
"We will go on as we have done so far. We have been brilliant. There is nothing to change.
"We are disappointed but nothing has changed, we need to keep on working hard and I am sure we get back stronger.
"We have to get away from this game now and get rid of it and think about the next one. We need to stay positive.
"We have still the league and the cup left and we are going to do everything to win them."
Celtic striker Odsonne Edouard levelled with a VAR-awarded penalty in the 82nd minute but Pep Biel restored the visitors' lead three minutes later before Dame N'Doye made it 3-1 to complete the scoring.
Elyounoussi insisted it was simply a bad night for Celtic despite being the better team on Thursday and in the first half in Denmark.
He said: "It was really disappointing. The fans were good, they deserved more, we deserved more.
"We played a great first half, created a lot of chances, I should have scored and in the end we were really disappointed of course.
"We shouldn't take all the credit from them. They were good, they were well organised.
"They didn't create anything except from that mistake and that gave them belief that they maybe didn't have before and that made it more difficult for us.
"We should have kept more composure, played as we did in the first half, finding the gaps and creating chances from there.
"But we started to stress a little bit and that helped them."