I agree with you that the defence hasn't looked quite as solid yet this season and I think there could be a few reasons for this.
Like Blade said we havn't had a regular right back yet really this season, with injuries/loss of form causing chopping and changing there. In 06/07 season, we had Gary playing there and in 07/08 Wes came in and looked like he'd been a regular there all his life. This year no one, apart from argeuably Rafael, has made a real claim for the position.
I think another reason as well for the defence not looking as solid is because last season the United back 4 were so dominating and impressive, that there wasn't really a lot of room for improvement. Wes, Rio, Vida and Evra all had probably their best seasons to date in a United shirt and that in itself was a recipe for sucess.
Having said that I still expect defensive performances to improve as the season goes on, but I don't think the defence can be singled out for everything that hasn't gone to plan this season. The strikers and midfield need to start converting more chances up the other end of the pitch, and give the defence some assurance that if they do make a mistake and lose a goal, that it won't cost us the match.
