Chelsea boss Claudio Ranieri pinpointed mistakes by his reshuffled defence as the reason for the home defeat by Besiktas.
Ranieri started with a three-man central defence of Marcel Desailly being flanked by John Terry and William Gallas.
But the move did not go to plan as Chelsea were 2-0 down after 29 minutes and failed to score against a resolute Besiktas back-line.
"We made mistakes and when you make mistakes in the Champions League you
concede a goal 99% of the time," said Ranieri.
"We started very well and tried to score a goal and we had two good chances.
"Then we made two mistakes and conceded two goals, and when you give away something to your opponents in the Champions League it is very difficult.
"My players tried everything to get a draw from the match but we faced an organised team in front of us who closed down all the space in the second half.
"It was difficult to create chances and whenever we shot we had a wall in front of us."
The defeat was Chelsea's first of the season and they were not helped when Celestine Babayaro was forced off injured and William Gallas had to play on with a knock because Ranieri had already made his three substitutions.
Ranieri added: "I am disappointed because I lost Celestine Babayaro and William Gallas through injury, and I lost the game, but I still feel good and I feel confident."
The loss leaves Chelsea third in Group G with Besiktas second and Lazio top.
Besiktas coach Mircea Lucescu believes his team took advantage of a Chelsea outfit still getting used to each other after a busy summer of player arrivals.
He said: "I saw a lot of Chelsea's matches last season and Ranieri did a lot of good
"But the new players have not yet integrated perfectly into this team, they make wrong passes because they don't know each other well yet, and the manager needs time because they haven't had much preparation together.
"Besiktas are a good team and it is not for nothing that we are playing at this level - we lost only once in our championship in 15 months, we are very disciplined and organised."