I don't think Karlsson makes sense for the Leafs, but at 5 mil I think he would fill a need for some team.
I wouldn't want to pay a lot for him, but as a change for the Leafs.... bring in Karlsson, move out Reilly in the offseason. If at $5 mil, it saves on some cap, gives us more offense... and while Karlsson isn't very good defensively, neither is Reilly. Finding a good defensive LD, to work with Karlsson, is probably easier than finding someone who works with Morgan.
As long as obtaining Karlsson, was a net zero cost, in moving out Reilly, I don't see the problem if we are looking to make changes.
That is, if Chychrun doesn't make it to UFA. If Chychrun makes it to UFA, I'd call that a priority off season signing, and then move our Reilly... same type of idea. He should cost around Reilly's cap hit, and then find a good LD to play with Chych. I'd expect him to extend in Washington though.
Expensive, but Provorov would be an excellent LD.
Provorov Karlsson/Chychryn
McCabe Tanev
OEL XXX/Timmins
Makes for an interesting D core.... if this franchise had the stomach to make real changes.
Edit... not exactly the most physical D core though... we'd probably want to bring someone in for that third pairing RD, that could bring that physicality.