Retegui was denied twice by Gunners goalkeeper David Raya after he failed from the rebound.
He later said: "I feel sorry for the lads, we played a great match. I missed the penalty, but you learn from your mistakes. Now I'm thinking about the next match. After the missed penalty, the whole team came to cheer me up, even the coach after leaving the pitch. We really wanted to give the three points to all the fans".
"The goalkeeper made a great save, but on the second ball I made a mistake, I didn't see him because he got up quickly.
"I didn't see the goalkeeper who rose quickly. When I saw that he rise I focused on the ball and I didn't see the goalkeeper. I'm very sorry, it was important to start with a victory.
"I just didn't see the goalkeeper..."