Don't know what I'm missing but Canha seems like a massive upgrade to the current DH production. Heck, put Yeli at DH and stick Canha in left and you've probably upgraded both sides of the ball.
And that is assuming nothing changes. I also trust the Rays ability to find and develop promising arms. My guess is they think they can fix something and turn a good pitcher into a very good one at the cost of a non-elite prospect.