Cardiff boss Malky Mackay was angered by referee Anthony Taylor's decision to allow the goal to stand, but Marshall has accepted that it was as much his responsibility as it was that of the official's.
"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."
Eto'o later got on the scoresheet himself, with Oscar and Hazard completing the second-half comeback.