The question is when can you decide as a team that the player has solidified their role and will not suddenly need a return to Triple-A in June to work on hitting sliders or perhaps to try moving to a new defensive spot. Just being a top prospect does not always assure you of a roster spot. Looking at Mike Trout you see someone who everyone knew nearly two years ago was here to stay, but the Angels and Trout choose this spring to reach an agreement.
If the Red Sox want to make an offer they would need to look at players signed in their first year or two and what value they would be worth. Just this week Jedd Gyorko signed an extension giving him five more years after this season at $35M and an option for 2020 at $13M. The comparison to Gyorko would surely be one the Red Sox would bring to the table if and when they talk to Xander Bogaerts as well as updating the Evan Longoria deal.