Plus, Lou Lamoriello has to convince Horvat to sign long-term. However, with over a month to go before the trade deadline, if the Islanders GM believes his team won't make the playoffs or that he won't be able to reach a long-term agreement with Horvat, he still has time to trade him, and that's been the assumption on Twitter since the trade.
Recall that the Islanders made this kind of move back in 2013, acquiring Thomas Vanek from the Sabres, only to trade him to Montreal with just minutes to go.