I think they need to continue to iron out their own game.
Everyone is assuming when they lose it's because of whatever bias they had before the season started (#2C or LD) but the #2c has done more offensive play driving 5on5 than any player on the team. And the defense's weak link in the first 11 games isn't any of the left siders, it's Carlo.
When you look at their With/Without Chart below, you see Hall and Coyle as the players furthest to the right which means they are creating a lot of quality scoring chances. They are also towards the bottom right which means they also give up some scoring chances (but I think that's a function of playing with Smith who's been a train wreck defensively this season). You'll also see Carlo as the player furthest to the left and below the red line which means he gives up more than he creates. Every other defenseman on the team is on the other side of that red line. Haula stands out as well. He is below that red line and still pulling big minutes. He has the 2nd worst plus/minus on the team, 1 point in 11 games and seems to be struggling mightily in his matchups and with the way Cassidy has deployed him.
If I'm Cassidy, the first thing I do is tinker with the 3rd line. They have to plug that hole if they expect the boat to float. The second thing I do is scale back Carlo's minutes and scale up Clifton. Carlo is clearly the better player, but he's not playing like it, and Clifton has played very well. Third I'd spend practice time working on defensive zone coverage. These miscues and missed reads that are happening (by forwards and defensemen) go all the way back to the Islander series. I mean, the goals they're giving up are nearly identical. Goals where a F has someone covered in front so the D leaves to confront the puck carrier, at the same time as the forward in front releases his man to go do the same thing leaving the guy in front uncovered. It's basic, it happens every game to every line and it's why it seems so easy for other teams to score.
They will still probably end up needing a RW for the 2nd line, but that's where I would start. There are a lot of new faces and there were bound to be growing pains. New guys learning the system, learning communication on the ice, learning each other's tendencies. It's a work in progress and will probably be that way until December.