Atletico struck the final three goals to edge this thriller, with Antoine Griezmann hitting the winner in injury-time.
Afterwards, Pimienta says he couldn't fault his players' efforts on the day.
He said, "They have taken a step forward with the changes that have subjected us much more, we have practically not been able to get out of our own half and we have not been able to create danger. We have been in our own goal almost all the time, they have created many goal-scoring opportunities and even so I think we have defended very well.
"Atleti's goals have also been authentic golazos . Griezmann's control is brutal, the shot from outside the area with three of our players very close and putting it so close to the post... these things can happen and I feel bad because the boys have made a brutal effort, I take my hat off to them.
"They have come here with personality , we have played very well in the first half, with goal-scoring opportunities, subjecting Atlético, but in the last half hour they have put us under a lot of pressure."
He also said: "I would like to highlight the whole team. There are people who were on the bench, who didn't take part and I feel really bad. A brutal effort with people from the house, who have this feeling. Kike Salas was crying, with the great game he had, because of the last action that wasn't his fault.
"Griezmann's performance is spectacular and when you play against players like that, anything can happen. They are youngsters who have an incredible future ahead of them and I feel bad that they didn't take anything from here because it would have been a nice reward for them."