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Real Madrid need to finish next quarter final with 3-0 scoreline to take the game to penalty shootouts. If Arsenal score one goal then Real Madrid has to score four goals to take the game to penalty shootouts. If Real Madrid directly want to qualify for semi finals without penalty shootouts then have to finish the scoreline 4-0. I don't think Arsenal will play that bad in next quarter final after giving there best in first leg. From my point of view, it will be worthy to place money on Arsenal for next leg then on Real Madrid.
There are several scenarios that could benefit Real Madrid in the second leg of the quarter-finals and anything is possible. Playing at home is a big advantage for Real Madrid, but given the large aggregate, Real Madrid's chances of advancing to the semi-finals are getting lower. I never said Real Madrid can't come back, but they probably won't be able to do it this season if Arsenal play as well as they did in the first leg in the second leg.
I don't think Arsenal will lose all their advantage in the second leg and let Real Madrid destroy all the chances, but we never know for sure. If Arsenal want to qualify, then they don't have to focus 100% on defense because Real Madrid can press better. Arsenal must play openly with a combination of defense and attack, if they only defend because they are already leading 0-3, then Real Madrid will come back.