Jose Mourinho's men had the ball in the back of the net within the first seven minutes, only to see the strike rulled out for offside, but the Spanish outfit still ran out 4-1 winners and it was their level of intensity that the youngster found hard to believe.
"The first five minutes I did not know what had hit me," he told De Telegraaf.
"The level they were playing at was way higher than that of PSV, who are probably the best team that we take on the Netherlands. It may sound crazy after a 4-1 defeat but this was, as a 18 year old player, a great experience."
Fischer also revealed that he blamed himself for allowing Karim Benzema to net an opener, which was subsequently ruled out for an offside.
"I blame myself, I lost the ball. I was shocked.
"Within 10 seconds of the ball leaving my foot the ball was in the back of the net. I said to myself 'Jesus Christ', but luckily Karim Benzema was just offside.".