I think Barcelona have no other choice in these circumstances and have to make the most of the players they have and Real Madrid they will be very superior in terms of players.
In terms of players and home advantage which is obvious. If Barcelona got Lewandowski in their lineup then they might have a shot but with their current lineup is just not enough to win. If they can pull a 2-1 lost or keep it draw on the 1st leg then they might have a shot on the 2nd leg though but Real Madrid will definitely pressure them starting from the 1st half whistle
The game of football is funny and the result in the game after first half is really a good one for Barcelona team which most people predicted not to win the match because of the absent of their star man Lewandoski. The only shot on target has come from the Barcelona side and they have managed to win 0:1 in the first half I believe they will end up winning the game if Real Madrid do not put much pressure on them. With the performance of Barcelona in the first half they may end up winning the game 1:2 which is the opposite of what you predicted.