The LaLiga side established a 1-0 advantage in the first leg of their Champions League round 16 tie at Estadio Metropolitano on Tuesday night through a fourth-minute goal by Saul Niguez.
"We knew it was going to be a proper battle tonight, especially here at their home ground with the fans behind them," the No.4 told BT Sport post-match.
“Obviously it didn't help that we conceded the goal from a corner, the first chance for them – not even a chance, a bit of luck. But it counts. It was tough.
“We had the ball most of the game but, unfortunately, we couldn't create massive opportunities. But we still have another 90 minutes to set it straight and we should be full of confidence still."