The United midfielder also insisted the return of England defenders Phil Jones and Chris Smalling, together with Nemanja Vidic's possible comeback from a knee injury against CFR Cluj in the Champions League on Wednesday, can only help United's cause.
Fletcher said: "Our defending is becoming a concern but I think, particularly at set pieces, it's individual mistakes, which we can try to sort out. We are talking about it and we're trying to look at it and put it right. The biggest thing is we keep coming back. It's a lot more disastrous when you don't."