I trade Tatar or Nyquist before Mantha. Not only does Mantha stand a very good chance at being the better player in the long run, he has much less trade value right now.
And with Mantha, Athanasiou, Pulkkinen, Bertuzzi, Jurco, etc., one of Tatar and Nyquist are expendable. I think one of them should be moved as the main piece in a package for a top tier defenseman.
One of Tatar/Nyquist (Can use kids to make up the difference)
One of Sproul/Marchenko (With the acquired D, plus Ouellet/Jensen waiting in the wings this loss wouldn't be felt at all)
One of Jurco/Pulkkinen (Again, with so much quality forward depth this loss likely wouldn't be felt much at all)
One of Svechnikov/Sheahan (Larkin, possibly Athanasiou, Holmstrom, plus whichever of Sheahan/Svechnikov isn't traded leaves us with plenty of centre depth down the road.
1st
A package like that would have to be enough to bring in a top-tier defensive talent. Shattenkirk, Ekman-Larsson, etc. Could possibly even try to pry Jones out of Nashville. They have tons of high-quality D but could use some high-quality forwards which we have in abundance.