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Former Porto defender Ricardo Costa insists Porto can still upset Chelsea.
The Blues won the first-leg 2-0 and are huge favourites heading into the second leg.
Costa said: “I always said and I reaffirm until the result on Tuesday tells me the opposite: I give 50/50 to each one, because both are playing this quarterfinal.
“Not having an audience in the stands and playing in a stadium that is neither yours nor Chelsea’s is rare and is a weird moment in football. Anything can happen and I believe that FC Porto can turn it around.
“For that, they need a lot of possession and to score goals, without letting the opponent go out in quick and vertical transitions.
“Chelsea will play as the personality of their coach, who is cautious, has principles of play and prepares his teams in detail. He will keep the three defenders, but when he plays in a low block, he will combine a defensive line of five with two midfielders.”