Nightengale added that clubs aren't willing to part with much since they can wait for him to become a free agent next offseason and offer a deal worth between $27 million and $30 million per year without parting with any prospects.
The 27-year-old Betts has spent his entire six-year MLB career in Boston thus far.
Betts is already a four-time All-Star, and he was named American League MVP two seasons ago. He has also finished eighth or better in the MVP voting in each of the past four campaigns, won four straight Gold Glove awards and three Silver Slugger awards.
His best all-around season came in 2018 when he hit a career-best .346 with 32 home runs, 80 RBI, 30 stolen bases and 129 runs scored. Betts also helped the Red Sox win the World Series that season, as they got past the Los Angeles Dodgers in five games.
Betts' production dropped off a bit last season, but he still managed to slash .295/.391/.524 with 29 homers, 80 RBI, 135 runs scored and 16 steals.
Given his status as a five-tool player, it can be argued that the only player in all of Major League Baseball who can match Betts in terms of an all-around skill set is Los Angeles Angels outfielder Mike Trout, who is a three-time MVP.
The Red Sox would undoubtedly love to keep Betts over the long haul, but with shortstop Xander Bogaerts, designated hitter J.D. Martinez and pitchers Chris Sale, David Price and Nathan Eovaldi all signed to big, long-term contracts, it may not make financial sense to do so.
Even so, trading Betts for anything less than a bevy of quality prospects would be a questionable move since there may be more value in attempting to make one last run at the World Series with Betts on the roster.
Boston went just 84-78 last season and missed the playoffs, but based on how much talent the team possesses, it isn't outside the realm of possibility that Betts could help the Red Sox make a deep postseason run again in 2020.