There is a big problem in that game. Atletico is not Manchester City man. They are better than Manchester City and with the way they are playing they might be the team to eliminated Real Madrid from the competition. Sometimes i get to pity for Real Madrid because they have faced stronger opponents than most of the team currently in the UCL. Even in difficult stages they get to play a very strong competitor.
If course there will be a big problem in the game but that will be if Atletico Madrid plays at their best, it will not be easy for Real Madrid not because it is the champions league or because Atletico Madrid is performing well this season but because it is a derby match and this fixture as always been know to be a very strong and competitive gam. Manchester City are not in good shape so it was easy for Madrid to win them but it will not be the same with Atletico Madrid. It will be a very tough match and a difficult one to predict but we all know how good Real Madrid is when it comes to the champions league so we cannot rule them out because Atletico Madrid are better this season.
Actually every match is a problem for any club, because they are required to be able to win, especially this is the knockout round where there will only be 2 meetings, if they fail then they will be eliminated. It's different when they were still in the league phase, because there were many opportunities with 8 matches, 2 times getting unsatisfactory results could still make it possible to qualify and in fact many clubs are like that, but it can't happen in the knockout round like this. Victory is the price they have to pay very hard.