Cardiff manager Malky Mackay says referee Anthony Taylor made a "horrendous error" by allowing Eden Hazard's first goal to stand in his team's
Samuel Eto'o kicked the ball away when Bluebirds keeper David Marshall bounced it - a foul according to Fifa rules.
Mackay said: "He bounces the ball. It's a huge mistake. A horrendous error."
Chelsea assistant first-team coach Steve Holland said: "The referee had a clear view and knows the rules."
Cardiff were leading 1-0 at the time, following Jordon Mutch's goal. But, after Eto'o nipped the ball away from Marshall, Hazard slid home to level.
Mackay added: "At the time, I didn't see but I've been into see the referee. Our goalkeeper bounced the ball. He saw the player Eto'o there. The referee is saying that, at the time, he heard from his linesman that he dropped the ball.
"He said immediately that if the goalkeeper bounces the ball, then it's a foul on him. They've got it wrong between them and it's cost us. We've gone in 1-1 at half-time instead of 1-0."
Marshall said he is "gutted" by the equaliser and believes it was the game's defining moment.
"Goals change games - it was a silly goal to lose," he said. I always bounce the ball before kicking the ball so [it's] probably my fault.
"It's just disappointing because we had a game plan and it was working up until then."
Eto'o, Oscar and Hazard each scored second-half goals but Chelsea manager Jose Mourinho was sent off and refused to attend his post-match media conference.