So put the shoe on the other foot. Why would the Bruins, a team with a defence just as bad as COL's, trade a dman for a winger?
First of all, NO ONE has a defense as bad as the Avs do. Not even close.
Second, I don't know Sweeney at all, but I would imagine that if he is entertaining moving a D-man for Landy, he has a backup plan for either bringing in another defensemen, or a replacement will be coming with Landy.