City are bottom of their Champions League group half way through and set to be eliminated before the knock out stage again.
"I don't think we're ready to win the Champions League," Mancini said. "If we say we're ready to win it, we're not honest. In the Champions League it's clear that incredible situations can happen in February and March but we've been drawn in a tough group this year, like last year.
"We're a good team but we're not ready in the Champions League, like the other teams. Chelsea tried for 10 years to win the Champions League.
"They were probably the best team in Europe for 10 years and they won it [last May] after 10 years, probably when they didn't deserve to. They deserved to win it three or four years before. The Champions League is like this. If we win our next three games, then fantastic … anything can happen. But the road is very long."