The Scottish international returned to the United side for their 4-3 win at Reading on Saturday, after which Sir Alex Ferguson berated his back-line - insisting they would 'get battered' if they continued to play like that.
And with the Manchester derby the centrepiece of this week's Super Sunday - Fletcher admits there will need to be some marked improvement before the match at the Etihad Stadium.
"Teams like City will definitely punish any slack defending, so we are going to have to be really on top of our game," said Fletcher.
"It is becoming a concern, although I think, particularly at set-pieces, it is individual mistakes which we can try and sort out.
"Everybody knows that, and sometimes you can really focus in on the bigger games and make sure mistakes don't happen.
"I think we saw last season how important derbies against City are. We lost twice and we lost the league. Those results were a major factor in that.
"So we don't want to give them any advantage, by them winning against us, and I think it is about time we got a result against them."
United could be bolstered by the return of captain Nemanja Vidic this week - which Fletcher admits would be a huge boost.
"He's the club captain and a fantastic defender, so to have somebody like that coming back is always a good sign," he said.
"It's also great that we've got Phil Jones and Chris Smalling fit again, all these things are definitely going to help us.
"We are talking about the defending and we are trying to look at it and put it right. But I think the biggest thing is that we do keep coming back. I know we keep saying that but it is a lot more disastrous when you don't come back."