Well, clearly Leafs didn't like what they saw, or, Lehtonen was possibly unhappy behind the scenes.
It also means they see something in this goalie that suggests he has the potential to be an affordable 1 or 2 moving forward next year, and/or the injury to Campbell is worse than we know. This will also free up cash for next year. If it was just a "hockey trade", this is a bit of a head scratcher taken on it's face.
Personally, I'm not a fan of light, offense only D-men which is what I saw with Lehtonen. Maybe I'm wrong. I prefer a Muzzin, Reilly, Brodie type with a shutdown pair somewhere in the six, over six Karlsons.
This probably also signals some other moves moving forward. If there was ever a year where Leafs ship out draft picks and prospects for a shot, this would be it. They have their nucleus signed for at least three more years, they have to be in "win-now mode", for this period of time.
The way I assess the situation, if they can find a hard forward and even another top six D, it would be prudent to pursue them. When you consider Simmons coming back at some point, that's a realized additional of three regular upgrades in the lineup for the playoffs.
With the depth they have on the Taxi Squad and in the minors, they would be prepared for a playoff run which everyone is demanding.