After winning the first leg 4-2 at Villa Park, they now have a two goal lead to defend at home.
"I believe the roles have not been reversed just because we had the first-leg result," a modest Mendilibar said when asked if Villa were no longer favorites.
"The qualification is after two matches, not one. What they have done in the first leg (coming from two goals down to tie the match at 2-2 before losing 4-2) they could do tomorrow.
They will be extremely serious to get qualification. Both clubs have their share of chances to win."
He added on opposite number Unai Emery: "Twenty years ago we (himself and Emery) first met as rivals.
The first time we got promoted to Primera Division, he was training Almeria and I was at Valladolid, we got promoted together. We have faced together many times at different clubs.
"But I think in most matches there has been a balance. I don't think one has a great lead. He has changed a lot. The fact he emphasizes a lot on details is key. He tried to take into account all details. What I truly I know about Emery is he 100 per cent believes he can knock us out. He truly wants that and that is a great weapon.
"Currently Emery believes he can knock us out. He believes he can get to the final, truly he will believe that. That is the weapon they have."