Hermida, 25, hit .259 with 13 home runs and 47 RBIs over 129 games with the Marlins in 2009 and set a career high with 56 walks. The left-handed hitter appeared in 81 games (73 starts) in right field and 51 (40 starts) in left. He made just one error in 205 total chances for a .995 fielding percentage, sixth among qualifying National League outfielders. He appeared in only three games after Aug. 31 because of an intercostal strain on his right side.
Hermida, Florida's first-round pick (11th overall) of the 2002 draft, has a .265 career batting average with 57 homers and 210 RBIs in 516 games.
"We still think there's a good hitter in there," general manager Theo Epstein said.
"Jeremy is a player who hasn't fulfilled his potential yet. We were able to acquire him at a reasonable cost to see if he can fulfill that potential with us."
Epstein cited Hermida's minor league numbers, age and the team's scouting reports among the reasons for optimism. "He's got offensive tools, a sound approach, a good swing," Epstein said. "He needs to figure things out to reach his potential. That may never happen but we have real good scouting reports and we like his approach."
Of Hermida's health, Epstein said, "That's one of the things that's held him back. Most of his issues have been soft-tissue related and nothing structural, nothing that should hold him back moving forward. We'll see what happens."
Asked if he projects Hermida as an everyday player, a star or a role player, Epstein said, "No one can answer that. When you acquire a player, sometimes you acquire them because you think there's a chance that with a change of scenery they'll reach their potential. We feel he may still have considerable promise."
Jones, 25, made his major league debut this season with Boston and appeared in 11 games in relief. The left-hander posted a 9.24 ERA with the Red Sox, compiling nine strikeouts in 12 2/3 innings. In 36 relief outings with Triple-A Pawtucket, he was 4-3 with two saves and a 4.25 ERA. The Red Sox signed Jones as an undrafted free agent out of the Cape Cod League.
The 20-year-old Alvarez combined to go 9-4 with a 2.26 ERA, 74 strikeouts and 16 walks in 26 games (12 starts) between Class A Salem and Short-A Lowell in 2009. He led the New York-Penn League with a 1.52 ERA while recording eight wins over 14 outings (12 starts) with Lowell. Signed by the Red Sox as a nondrafted free agent on July 2, 2005, Alvarez has compiled a 23-15 record and a 3.21 ERA in 76 career minor league appearances (48 starts) in the Red Sox system.
ESPN's Peter Gammons and ESPNBoston.com's Steve Richards contributed to this report.