Samuel Eto'o stole possession from Marshall as he bounced the ball to help set-up Chelsea's equaliser for Eden Hazard before half-time to cancel out Jordon Mutch's opener for Cardiff.
Cardiff boss Malky Mackay was left seething by Chelsea's leveller feeling referee Anthony Taylor got the call wrong in what was the turning point in the game.
Scotland international Marshall admitted he was gutted at the decision feeling he had let his team-mates down.
"I am not sure if the linesman, referee think maybe I have dropped the ball thinking he's [Eto'o] not there, but I knew he was there and I have bounced it so hopefully we can get it cleared up," Marshall told Sky Sports News.
"But it doesn't help us now, it is just disappointing as the boys had done great in front of me and we were in such a good position.
"I am just gutted to let them down.
"As I say I shouldn't have bounced it and it probably is my fault."