In search of a solution to Toronto's defense, I decided to pay corsica.com a visit. In case any of you guys don't know what that is, it's a website for advanced stats that are based mainly on corsi-for and corsi-against which is a measure of how many shots-for you are on the ice for and how many shots-against you are on the ice for. I filtered my results to just defense-men and the results were that the LA Kings have some very very good defense-men. The order of the best defense-men according to corsi-for percentage were Jake Muzzin, Drew Doughty, Brayden McNabb, Anton Stralman, and Josh Manson was fifth. Out of these top-5 the only player that Toronto would be able to get in a trade would be Manson because the other 4 are too valuable to their team as top 2/3 defense-men, whereas Manson is more of a fifth on the Ducks team.
The reason the Ducks would be open to trade him is because otherwise they'd have 5 defense-men to protect from Las Vegas. Those 5 being Kevin Bieksa (NMC), Sami Vatanen, Hampus Lindholm, Cam Fowler, and Josh Manson if they don't trade him. Now, we've all heard the Lindholm and Fowler rumors but nothing have come out of those rumors because Anaheim's asking price is too high as it should be with two talented young defense-men. However, Manson is a different story as he is older than both of them, although only a couple months older than Fowler. The reason Manson is the odd one out is because 1. Lindholm is the future of Anaheim's D corps and 2. Fowler is a much better offensively-gifted defense-man. One thing to note is that Manson is actually tied with Bieksa for Anaheim's second most used defense-men in terms of total time on ice, and tied for third in terms of time on ice per game. As you can see both Fowler and Lindholm are used more than Manson, and Bieksa is used as much as him which says something about the value he has to Anaheim as a team.
Now, I didn't wanna base my whole reasoning for why we should trade for him on how Anaheim uses him in games so I looked for Ducks bloggers to see what they say about him, and eventually I found a forum specifically about what he's worth. Here's the link to it if you would like to see: http://hfboards.hockeysfuture.com/showthread.php?t=2140045. Basically, what most of the people are arguing is that he's a top-4 defense-man on a very cheap contract, but what they fail to recognize is that he won't be able to get protected come the expansion draft this summer, and that's why they don't realize the need to trade him.
Although Anaheim would love to trade Bieksa instead of any of the defense-men mentioned above, they cannot because he has a full no-movement clause in his contract, and at this point in his life and career I don't think he'd like to be traded from a contender and have to move his family. Manson seems like the odd one out based on his age, usage, and the fact that the expansion draft is coming up. He'd also be a perfect fit on the top-pairing with Morgan Rielly as he's a right-hand shot as opposed to a left-shot in Rielly.
On to what would be a fair offer for Manson. I don't wanna get into hypothetical trades because there can be so many of them but what I will say is that the Leafs would have to be willing to take some salary in the trade in the form of most likely Clayton Stoner who has a cap hit of 3.25 million for this year and the next. In return the Leafs would need to give up a B level prospect (Bracco, Soshnikov, Rychel, Leipsic, Kapanen although I think he's a stud, or Timashov maybe), a pick or two in the 2nd-3rd round, and depth pieces with expiring contracts to even out the salaries as both teams are up to the cap. Hypothetically, Bracco+2nd+3rd+Polak+Hunwick/Marincin/Corrado for Manson+Stoner would possibly do it. The closest recent example would be the Sabres and Panthers trade which involved Kulikov+2nd rounder for Mark Pysyk+2nd rounder+3rd rounder. Obviously, Kulikov is not as as valuable as Manson but he is still viewed as a top-4 defense-men and the return for him was a 5th d-man and a 3rd rounder (cancelling out the 2nd rounders). In the trade I hypothesized above Toronto is essentially swapping bottom pairing defense-men in Polak and Stoner. The other part of the trade is Bracco+2nd+3rd+another depth defense-men for Josh Manson, which seems pretty fair to me.