Why wouldn't the Sox urge Harper to let them know, as soon as possible, in order to give them a chance to seek additional players, and thus improve their prospects for a competitive 2019? That would certainly be in Harper's interest, if he is seriously considering coming to the Sox. A team that is already prepared for a post season run, and only needs to acquire one more piece, would not be able to legitimately make that plea, but the Sox have other needs. If the front office simply told him that they want to win now, and are willing to commit the financial resources to pursue additional players, it could be to everyone's advantage to wrap up their deal, as quickly as possible.
I like that approach better than a "hard ball" tactic, such as; "Hey, here's our offer. If you don't take it now, we're moving on. We can't sit around and wait on you, at the risk of losing out on other potential acquisitions." In other words, there is indeed a legitimate reason to inject some urgency here, and urgency helps close the deal.