Chris Martin and Johnny Russell struck in the space of three first-half minutes to deservedly put Derby on course for their first Sky Bet Championship home win of the season.
"You are disappointed when you lose the first goals which we have done too many times this season," said McDermott.
"In the last two games we have let ourselves down away from home but before I turned up here I think we had 21 games away from home and won one.
"We need to get a mentality away from home where we are very difficult to play against and we weren't today. This has recurred too many times, giving away the first goal and it's something we need to look at.
"We played a lot of football today in the middle of the pitch but you are judged on what you do in both boxes and we weren't good enough in both boxes.
"We need a winning mentality. When I was at Reading two years ago when we hardly had any points on the board after 10 games and we just clicked into place and we worked on that.
"We've had that conversation in the dressing room today and talked about that mentality and it's not about hoping you will get a result, it's knowing you will get a result and that's where we have to get to."
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