Celtic had to be near immaculate against Atletico even to contemplate a draw in Madrid. However, losing a very early goal meant it would always be an uphill battle.
However, an unjust and bitter blow handed down by UEFA and their referee meant the task was made almost impossible. Daizen Maeda has made an honest tackle, the two players have gone in for the ball, and Maeda has caught the Atleti player in the leg.

A yellow card at most. However, the theatrics by Madrid and the moaning paid off when the referee was told to go look at VAR. instead of judging the tackle by the whole phase of play, they zoomed in on one screenshot and sent Maeda off.
Celtic down to ten men and gave the home side who already have a massive advantage, another helping hand.
Was the decision to give Daizen a yellow a clear and obvious error? Not a chance!