Lyon qualified in second place of Group F, finishing behind Manchester City, and have been rewarded with a last-16 tie against Barca.
But Les Gones are now turning their attention to the Champions League, and Cornet - who scored three goals across two games against City in the group stages - told Goal: "Barca are... they're something else.
"There is Messi, Suarez, Coutinho. It will be necessary to negotiate it well. We have to be ready for D-Day!"
Barcelona have been in flying form this term, topping La Liga and only dropping four points in their Champions League group.
Asked what Lyon need to do to stop the Catalans, Cornet said: "Block Messi! I laugh about it but it's what we will have to do.
"It will take a lot of defensive work at first, be all the time concentrated and take the few chances we could have going forward.
"We know that in this kind of match that you cannot waste chances, so it will be up to us to be effective in front of their goal."
Lyon's job will be made harder due to the absence of club captain Nabil Fekir due to suspension, and Cornet admits the forward will be a big loss.
"Nabil won't be here and everyone knows the importance of this player, whether to boost the guys before the match or make differences on the pitch after," he continued.
"But there is not much to say. He will not be here, we will have to do without him and we have the players for that."
Despite Fekir's absence, Cornet still believes OL have considerable quality amongst their ranks and singled out goalkeeper Anthony Lopes as having an important role to play.
"For me, Anthony is one of the best goalkeepers in Europe at the moment. He is important in the locker room because of his experience. He is someone who also transmits his winning mentality and it is important."
"Houssem [Aouar] is subtlety, finesse and talent. When you play with two midfielders like that [with Tanguy Ndombele], it's easy.
"Tanguy has an exceptional technical quality, he breaks the lines and as an attacker it's a great thing to play with him. The key can be there, in this area of play, but it will be above all about teamwork.
"We have the players. Whether it is Memphis [Depay], Berto [Traore], Martin [Terrier] or me, we have the players. Afterwards, you have to see what the coach will put in place. We are 100 per cent focused on Tuesday's game and we will study everything closely."