Mariners general manager Jerry Dipoto said last week Heredia dealt with the shoulder issue much of last season and had to have the dislocated shoulder put back in place on at least four occasions, which likely affected his offensive performance.
"Guillermo's an exceptional defender," Dipoto said. "It's really hard to truly assess his offensive performance this year. This guy was playing with a dislocated shoulder virtually all year long and never complained.
"He had four different instances where it had to be put back in place. He was doing it himself, which I guess when you battle as hard in life to get where Guillermo ultimately ended up, he fought long and hard to leave Cuba to come to the United States to make it as a big league player and he did it. He didn't want to give that up."
Heredia made the 25-man Opening Day roster after a strong spring and was hitting .286 with six home runs and 22 RBIs in 90 games through Aug. 18, before batting just .135 in his final 33 games of the year while being called on to play center field every day after Jarrod Dyson's sports hernia injury.
Heredia finished with a .249/.315/.337 line with 43 runs, six homers and 24 RBIs in 386 at-bats.
"At the All-Star break, he was quite fine with the bat in his hands, so I don't want to overreact to the offensive struggles he had in the second half," Dipoto said.
Heredia will begin rehabilitation immediately and is expected to compete for the starting center field role next season.
Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011. Follow him on Twitter @GregJohnsMLB. This story was not subject to the approval of Major League Baseball or its clubs.