The realistic issue with JBJr is what happens over the winter of 2019-2020. First, you will get the benefit of a full season to see if the hot streak or any improved consistency at bat comes on or not. Second , there may only be a limited chance of a July deadline trade if the Sox are out of contention and someone in the NL wants a defensive Cf'er. Third, DD will need to consider what Jackie's arb $# will be for next year . JBJr will likely be asking $10MM and the Sox may value him at less than 2019's $8+ million. He's also not that young. Next April he turns 30 and can expect an overall decline to be happening within a couple years. Who would offer him more than 3 years in any case ?
Unless he makes a significant step forward at the plate, while continuing his defensive excellence , I could picture the Red Sox FO hardballing Jackie after this season to the point of moving him along due to very small upside after 30. But he can throw a ball 400', so there's a chance that Heath Hembree's flyballs can be retrieved.