Roma surrendered a 2-0 lead to lose 3-2.
"We did very well in the first half, but after the break we settled and allowed our opponents to control the game," said Zeman.
"We made three mistakes on their goals and helped Bologna to get back into it. It is a heavy blow for the squad, as we must always be concentrated to get the best result.
"The mistakes came because we were passive in the second half. We allowed Bologna to play and never really went towards their goal.
"It's easy to blame the break for international duty, but that's not true. The players do not suddenly forget movements when they are somewhere else for 10 days.
"My concept is to attack, therefore keeping our opponents away from our goal. Today we did not do that."