The Cherries ran riot to win 5-1 on Saturday, with Dango Outtara hitting a hat-trick.
“Of course it is tough to take. I think it surprised everybody, especially the way we started the game. It was bad. It was bad. From the beginning until the end, it was a bad day,” Nuno said.
“It is realising how we do things; how we’ve been playing football until today. It is clear our strengths - we are a team that needs to be close to each other, compact and don’t allow space for our opponents to drive and make their actions.
“The moment that we lose that aspect of the game, it becomes much harder for us. Much, much harder.
“Not only today. The second half against Southampton was a warning. It is a 7-0 that we have in 135 minutes, that’s the reality. We have to take a deep look at how we did it today so we cannot repeat it again.
“It must be a warning for everybody - us as a group of coaching staff, the players, the fans. Realising how we do things, and we can never abandon those aspects of the game.”