if I had to guess, it's just that they want to keep him and so will not trade him until right at the deadline.
So we can't make any assumptions about how close an extension is. Even if it seems hopeless, I'd imagine that Hertl is at least listening. You have to wonder if maybe Karlsson's timing at coming back is partly to show Hertl that the team isn't aged out and has some years of competition left in it. It might be a good sell if they can get a few wins like they did last night. But, if I'm Hertl I'm looking at July like a huge opportunity to join a team on the rise like the Rangers.