With the Rome derby looming, Petkovic feels his players weren't on the case.
"I consider myself the first person responsible for this defeat," said the Coach. "Up until the first goal we weren't doing too badly, but our attitude after the second was inexplicable.
"I just hope the derby was not already in the minds of my players. It is an important game, but it's still a week away.
"I substituted Hernanes at half-time because he was not playing well. We kept up the tempo, but did not perform. It's a psychological issue.
"The Rome derby is very important, but before that there is the Europa League."