There's some expansion draft wrinkles that tie into a Hayes move. They have to leave 2 forwards unprotected that are under contract in 17/18, and played at least 40 games this season or 70 combined between 15/17 and 16/17. They can protect 7 F. Assuming no further moves, Bergeron, Marchand, Pastrnak, Backes, Krejci, Spooner and Beleskey are likely the 7 protected forwards. Hayes meets the exposure requirements, as does Riley Nash. They don't have anyone else. Trading Hayes means they'd have to find another forward that satisfies the exposure requirements. That's either trading someone or signing Moore for another season.
I think the Bruins try to sneak Hayes through waivers and send him to Providence sometime soon, probably when Vatrano and Acciari are ready to return. There is some risk there because there are several teams who currently don't have the exposure requirement fulfilled, but teams are capped out and trying to win so he is more likely to clear now as opposed to later in the year.