Athletic were beaten on Thursday night, with Eldor Shomurudov grabbing the winner for Roma.
Before then, Nico Williams had put Athletic ahead before the hosts drew level through Angelino. Athletic then had Yeray Alvarez sent off, before Shomurudov eventually struck to hand Roma the advantage.
“I don’t want to talk about the referee,” Valverde began at his press conference.
“He did start handing out cards a bit too early and easily, but I have nothing more to add to that.”
He continued: “We are strong at home and I hope we can have half the atmosphere they had tonight. It was an extraordinary atmosphere, so passionate. We still hope we can turn it around.
“It’s true that Roma are a good team, they have players like Paulo Dybala, Artem Dovbyk and Matias Soule who can change a game, but we are still playing the second leg on our home turf.
“As soon as we got into the locker room, we were already starting to think about the return match at the San Mames. All we can do is win in our stadium.”
Shomurodov's goal was virtually the last action of the game, leaving Valverde frustrated.
“There is never a good moment to concede a goal, but when we scored, they started to push constantly. We had it all under control, except for when we were a man down and they found the decisive move. When a game is balanced, there is never a good time to concede.”
The Athletic coach concluded: “Roma are a strong team, they are not weak. When you are repeatedly forced back, you tend to just kick the ball away.
“Something didn’t work tonight, but we must look forward.”