Jon Guridi struck midway through the second-half for Alaves' victory.
Arrasate later said: "We are upset because I think we were not good at all. It was even, but we created very little in attack. It seemed like the point was a good prize because we were not creating much more, but an unfortunate play tipped the balance in their favour.
“In the second half we didn't get many chances, but we played more in our half. With the changes we thought more about keeping the ball, but we couldn't. After they scored the goal against us, the game plan changed.
“We expected this type of game. They are at a stage where they have taken a little less risk than on other occasions. Defensively we have not suffered too much, but we usually create more in attack and today we did not do that. The play that led to the goal is unfortunate.”