Isco was part of the team that lost 2-1 to Serbia in the second leg of their playoff last night, with the elimination also meaning that La Roja will not compete at the 2016 Olympic Games.
"I'm pretty annoyed about it. It was very hard to take because we all had our hopes and dreams, but everything ends here for such a fantastic generation," he told reporters.
"We all wanted to go to the Games but it's no use talking about that now, although we had no luck today.
"Their first goal was a rebound you don't often see and from that moment we were up against it. With the players we had it should have been enough to win, but they played their own game and took their chances."
Spain are the defending champions of the Under-21 European Championship.