I am also managing Man Utd. The problem is the marking. A lot of time, the center backs chase after the ball leaving the opposing team strikers unmarked.do you look at concentration stat? a lot of people ignore it but vital for defenders! especially if u are doing most of the attacking. Also try solid fullbacks rather than wingback type. eg when i had a league with utd if at home id usually use rafael at rb but if away to tricky team id use phil jones. Also look for speed so can recover.