The 19-year-old left back has been brought off the bench a number of times this season by Ange Postecoglou but being trusted to play the full 90 minutes against the La Liga outfit was his manager's biggest show of faith to date.
Despite the 4-3 defeat, the academy graduate showed no fear and earned plaudits for his performance in what was only his second Celtic start, his first coming towards the end of last season in a 4-0 win over St Johnstone.
“Running out to start the game, to play against Real Betis in that amazing stadium all felt a bit surreal," Montgomery said.
“When you are coming up through all the age levels, it is the thought of playing in big games in these sort of surroundings that inspires you.
“It is something you dream about for years and years so actually be doing it takes a bit of adjustment.
“I had been involved against Midtjylland and Alkmaar in the previous rounds so I had experience of European football.
“Betis is another level, though. They are one of the teams you see when you are watching La Liga, competing with the likes of Real Madrid and Barcelona.
“You are there to do a job, though, and it was fantastic to be given a start.
“The manager told me just to be myself, to forget the occasion and focus on playing my normal game so that is exactly what I did.
“I think I did okay too in terms of that. I tried to get on the ball whenever I could and make good use of it when I did."
The teenager could be called upon again at Livingston today as he stakes a claim for the left-back jersey ahead of the injured Greg Taylor and recent arrival Liam Scales.
“Livingston will be a very different challenge from the one we had in Spain,” he said. “I know quite a few don’t like the plastic pitches but it is not something that bothers me at all.
“I played on artificial surfaces coming through the academy at Celtic and always enjoyed playing on them just as I would on grass.”