Think there are a few things going on here IMO.
A lot of teams have copped on to the right side of our defence being the more vulnerable, particularly with Matip being out. Would you rather attack Robbo & VVD or Trent & Lovren/Gomez?
Trent does get caught out a lot, especially to the diagonal ball over his head.
VVD has not been as commanding as he was last season. Whether this is down to a natural drop-off due to being so good last season, or he's maybe lording it a bit on the back of all the accolades he's received, I'm not sure. But he has been caught looking over his shoulder a few times in the area this season at an opposing striker who's scoring.
Defending free-kicks we've also been more vulnerable. Against City, they broke the offside trap twice and missed 2 virtual sitters, fortunately for us. It seems the line we hold and step-up is not as well timed or organised as it was last season. I think VVD is the man to organise this, so again, he needs to sharpen up here.
As long as we keep scoring goals, this isn't an issue, but once we hit a lean spell up front, we need the defence/DMs to be right on the money. I'm sure they will be when the time comes.