"We started well. We tried to score. Then we made two mistakes for the goals," Ranieri said after his side were sunk 2-0 in Group G by two first-half goals from Sergen Yalcin.
"When you make mistakes in Champions League it's a goal, 99 percent of the time it's a goal."
Chelsea piled on the pressure in the second half against Besiktas, who were reduced to 10 men when Ilhan Mansiz was dismissed on 51 minutes, but failed to find a finishing touch.
"It was like there was a wall in front of us," the Italian said.
Ranieri praised Besiktas as a well organised side. At half time he brought on Damien Duff and Jimmy Floyd Hasselbaink to try to make extra width but Besiktas closed them down.
"They did not give us any space. When a team close all the space like Besiktas there is not much you can do."
Chelsea captain Marcel Desailly, whose stumble contributed to the second goal, said he was very frustrated his side did not manage to score.
"We will have to work hard and learn more and keep listening to the manager," Desailly, who has won two Champions League trophies (with Marseille and AC Milan) as well as the World Cup and European Championship with France, added.
Besiktas's Romanian coach Mircea Lucescu said he had observed Chelsea closely and had seen they needed more time to gel. "They do not all know what each other is doing," he said.
"We covered space well. We blocked wings," he said.
"Besiktas is a good team. It's not by chance that we have reached this level.
"But in the last 10 minutes we were lucky not to concede a goal with all those crosses coming in."
Goalscorer Sergen said Lucescu, who knows Ranieri well having worked in Italy at the same time, had told the side exactly how to play.
"We tried to do what the boss said and we succeeded," he said
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