Thanks to John Obi Mikel I think we have a better excuse for Chelsea's defensive woes than the generic high line or Jose Bosingwa issues. According to the Nigerian midfielder, Andre Villas-Boas is attempting to implement a zonal marking system, and the Blues are still working on adjusting to it properly:
We talked about [zonal marking] at Cobham, but it is early days to change whatever we have started. We need to stick with it and work at it, the manager knows what is best and wants the best. Whatever decision he takes we have to go with it. Hopefully we can get better with it, we never did the zonal marking so much before, but that's what he wants to do and we need to work on it.
-John Obi Mikel. Source: ChelseaFC.com.
Zonal marking, in case you're not aware, differs from man marking in that players don't take responsibility for their opposite numbers, in case owning a specific area of the field. Zonal marking allows for several benefits, mostly in terms of retaining a defensive shape, but there are also weaknesses in terms of tracking runs and experiencing overloads in certain zones, especially if the team isn't experienced with the system.
Considering zonal marking is considered a more modern system, it's a little bit surprising that Chelsea haven't really being pursuing it - you'd have thought that Jose Mourinho would have made use of it, even if the likes of Carlo Ancelotti didn't. I'm not sold on zonal vs. man marking, but apparently Villas-Boas is and that's good enough for me. However, in order to go to a proper zonal setup, there are going to be some growing pains, and I think we're experiencing them now, especially on set pieces.
At least there's an explanation now.