Real Madrid kicked off the post-Cristiano Ronaldo era with a 4-2 defeat at the hands of Atletico Madrid in Tallinn, Estonia in the 2018 UEFA Super Cup. Diego Costa scored the fastest goal in Super Cup history and put Atleti in front after just 49 seconds.Karim Benzema's header at the far post brought Real back on level terms just before the half-hour mark. Atletico right-back Juanfran was penalized for sticking his arm in the box and the resulting spot-kick was tucked away coolly by Sergio Ramos to put his side in front at 2-1Diego Costa started Atleti's comeback and equalized in the 79th minute to force extra time. Two quickfire goals by Saul Niguez and Koke wrapped up a dramatic Super Cup tie and Diego Simeone's side lifted their third Super Cup trophy in the last eight years.On that note, here are the 3 reasons why Real Madrid lost against their city rivals:#3 Asensio has a poor outingAsensio was a let-down for Real MadridWith Cristiano Ronaldo gone, the left-wing is up for grabs at Real Madrid this season and Marco Asensio will be one of the favourites to occupy that vacant position week in and week out if the Champions League holders do not buy a new left-winger until the closing of the transfer window.However, Wednesday Evening in Estonia wasn't Asensio's night as the Spaniard had a disappointing game and was substituted after just 56 minutes. The former Mallorca man had very little to show for and failed to contribute anything significant during his short stay on the pitch.Asensio was below par and it affected his game as he failed to supply any killer passes or crosses into the box. There were instances when there was an opening but he squandered those glorious opportunities which could have put Real in front by a goal or two.Luka Modric was benched for the clash against Atletico Madrid before he came on as a substitute. However, Julen Lopetegui may revert back to the 4-3-3 system on the weekend which could see Marco Asensio lose his place in the starting lineup.Asensio will have to be more prolific if he aims to nail down his spot in the playing XI under the new manager and he will have to capitalize on the chances he receives as a starter to establish himself as a full-time starter at Real Madrid in the long run.