The London side slipped from the top of the Premier League after conceding two soft goals from corners against Villa.
"Yesterday we worked on this particular situation," coach Carlo Ancelotti told reporters on Tuesday.
"It's difficult to train for set plays just like it's difficult to train for penalty kicks. But we worked on it because we have to improve.
"You can play a good match but if you don't pay attention once for six seconds you can get a bad result," the Italian added.
"We have no problem criticising each other in the dressing room," the midfielder said. "With a good team and strong players they have to do this. And we did it. After we shake hands and go back on the training pitch and get on with it."
"It's one of our strengths that we are very well organised so it's a little bit unusual for us to concede two goals in one game," he said.
"It's all about individual mistakes and that happens... You can train (for) many things but if somebody sleeps in a situation you concede a goal so everybody has to be aware of this."
"It's a very dangerous game because Atletico have not started so well (in Europe) and need points," Ballack said.
"But we are a strong team, we've shown we can reach the semi-finals and we believe in ourselves."
"They have very good quality above all in the forward line with (Sergio) Aguero and (Diego) Forlan. We have to pay attention in dangerous areas. Aguero can change a match with his quality," Ancelotti said.
Chelsea will be without in-form front man Didier Drogba, serving the last of a three-match ban for an outburst after last April's semi-final defeat by Barcelona.
Asked about Chelsea's defensive frailties, Uruguayan Forlan, a former Manchester United striker, said he was expecting nothing but quality in the penalty area.
"They have good power in the air and players who are very strong. Sometimes you make mistakes in football, but it doesn't mean they are going to struggle in that situation."
