Outfielders:
Yasiel Puig:
After spending 6 seasons with the Dodgers, Yasiel Puig was traded to the Reds before the final season on his contract. While he has struggled this season, he was good in both 2017 and 2018 and would provide an upgrade over Jackie Bradley Jr., and likely has more upside. Puig may not be able to play center, which would mean one of Betts or Benintendi would have to move there.
He provides a more reliable bat than Bradley Jr. and has a solid glove. While so far, he has not been significantly better than Bradley Jr., he has had more recent success and is likely a safer option.
Adam Jones:
The Orioles longtime center fielder left for Arizona this season after the Orioles decided they wanted to go a different direction. So far, he has thrived in Arizona as Jones currently has a 109 wRC+ this season.
He can no longer play center as he only plays right in Arizona, so much like Puig, his acquisition would require Betts or Benintendi to move spots. It is unclear if he keeps this offensive production, but he would be a cheap upgrade over Bradley Jr.
Alex Gordon:
Gordon, the longtime Royal has always been known for his elite defense, but this year his bat has been good, posting a 129 wRC+ through 63 games. While his left field defense is elite, his centerfield defense is not.
The Red Sox would likely be better off moving Benintendi or Betts to center. He provides a clear upgrade over Bradley Jr. and would add another solid bat to the Red Sox lineup who also has a solid glove.