Boston projects to win their division. If so, the pick will be one of the last few of the round even without Boston making the Conference Final. If they can get at least a 2nd for Tarasenko, while dropping from 26-32 to the middle or end of the second round wouldn't be a wash, they at least recoup something. If they can get a 1st for Tarasenko, then they recoup the pick they give up for Tanev.
If it is a hockey trade, the only player who would make sense would be Joseph or Batherson. Calgary might need a Dube replacement, and they are also apparently shopping Lindholm. They are in a weird position, because they are still fighting for a wild card spot. So a hockey trade for a controllable asset might be appealing.
Obviously, no one here is going to want to move Joseph or Batherson for Tanev, let alone Tanev without an extension.
Hypothetically, if it is Joseph for Tanev, and he doesn't re-sign, they could flip Tanev at the deadline for a similar return to what Joseph might have gotten them. Of course, there would be injury risks.
The other option would be Tarasenko for Tanev, but I highly doubt Tarasenko would waive to go to Calgary.