The Red Sox have targeted Sandoval since the beginning of free agency. He arrived in Boston on Monday night, had dinner with slugger David Ortiz, and according to a club source, met with team officials Tuesday before leaving the next day.
The San Francisco Giants, San Diego Padres and Toronto Blue Jays also have interest in the 28-year-old switch-hitter, but a major league source indicated to the Globe on Friday that the Red Sox's offer might be in the lead.
Sandoval would represent a significant offensive upgrade at third base for Boston. A career .294 hitter, he hit 16 home runs with 73 RBIs and 68 runs scored this past season for San Francisco.
Sandoval played a huge role in the Giants' 2012 and 2014 World Series title runs, but the team did not discourage him from testing his value on the free-agent market. But they've also stayed involved in the bidding this offseason, with assistant general manager Bobby Evans telling the San Francisco Chronicle on Thursday: "I think the interest is sincere. I think Pablo loves this fan base as much as any player loves a fan base, and that could go a long way."
The Red Sox have the financial flexibility under the luxury tax threshold to handle a five-year, $95 million contract. But given the weight issues that have saddled Sandoval since 2010, the club isn't expected to display much enthusiasm for the six-year deal that agent Gustavo Vasquez said his client wants.
Information from ESPNBoston.com's Gordon Edes was used in this report.