The big defender was caught pushing up for offside to allow Eidur Gudjohnsen to race away and make it 2-0 in Sunday's FA Cup quarter-final.
Chelsea went on to win 4-0 and dash our dreams of a cup final double.
'Deano', disappointment etched across his face, insisted: "We had the chances, Les hit the post and if that goes in, it might have changed things but the second goal really killed us.
"I've not seen it yet and I'm not sure, some people say he was on, some say he was off, it was close call but it was a bad decision from me at the end of the day.
"I was confident he would be offside but perhaps I shouldn't have played offside that close to the halfway line because he's then through on goal.
"I'm disappointed because I made the wrong decision there. We gifted them four goals as well.
"We let ourselves down. We've not played to our potential and given four bad goals away."
Deano said it would be tough to now go and face the Blues again at Stamford Bridge in the Premiership on Wednesday - but insisted the lads were determined to put things right.
"Hopefully we can put this behind us," he added. "We've got to go there, do ourselves justice and put this result to bed.
"There is nothing we can do now but look forward, the next game is Wednesday and we've got to put a good performance on and get a positive result."
Dean Richards held his hands up for Chelsea's crucial second goal and admitted: "I made a wrong decision."