The La Liga giants are safely through to the semi-finals of this season's competition, but only after suffering a scare on German soil.
Real travelled to Borussia Dortmund on Tuesday holding a commanding 3-0 lead from the first leg of their last-eight encounter.
They were made to sweat, though, as Marco Reus netted twice for the 2013 finalists to leave Carlo Ancelotti's side hanging on.
Modric is at a loss to explain what happened to Real, as their standards slipped alarmingly, and admits there can be no more excuses as a star-studded outfit chase down a 10th continental crown.
The Croatian playmaker said in Marca: "I don't know what happened.
"It may be that we were complacent because we won 3-0 and we started thinking it would be easy. I don't know.
"We need to learn from this match. We always say we need to learn, but we never do.
"Of course, we're happy we've reached the semi-finals but we aren't happy with how we've played, especially in the first half.
"We can't allow what happened in the first half. But we improved and we are in the semi-final."