I realize it may create a log jam at left wing for you, but Tanguay has been a pretty effective left wing on the second line in Colorado. Jackman is a rental, and I don't think you would be able to get a much better return for him than Tanguay.
I think that The Blues can get a low 1st rounder from a team whose defence is decimated, or, perhaps a 2nd rounder and a 2nd tier prospect forward. I'd rather receive those in this deep draft year, than Tanguay alone.
IF you can get a low first rounder for him, what are the chances they would be a gifted pt per game player? With Tanguay, you know what you are getting and he is young enough that you will be getting years of play from him.