The Reds were defeated 1-0 in the away leg, but Reina is confident they can see off Athletico Madrid at Anfield on Thursday.
"It's not the best result. We didn't play well but we believed we could score here. It wasn't possible, but it's only half-time and there are still 90 minutes to play at Anfield," said the keeper.
"We knew they were going to push from the beginning because at the Calderon they are a strong team.
"We knew it could be a tough beginning, but after 20 minutes I think we settled down better and they didn't create too much apart from a chance in the second half.
"We cannot be happy, but it could have been worse."