Villarreal won 2-1, with the winner coming in the 94th minute through Ayoze Perez.
Pimienta later said: "I want to congratulate Villarreal because they have been more effective than us. This is about generating chances and scoring them. We have to correct things in defence because by playing the game we played today, not only did we not win but we lost, it is to be worried.
"It is unfair that with three arrivals that Villarreal has they end up winning the game. I think this is the game, but we have to be more consistent and that way it is difficult to score points.
"It can't be that they score two goals in three or four arrivals."
He added: "We played a very complete match in general terms and we can't possibly lose it. If they score four goals in two games it means we're not doing something right."