The Red Sox have had several great left fielders in their storied history, such as Ted Williams, Carl Yastrzemski, Jim Rice, and Manny Ramirez (hey, I didn't nescessarily mean defensively). One of the most underrated players in my opinion is Mike Greenwell. Greenwell had some great years in his 12 year career, all of which was spent with the Sox. I took a look at his stats and this is what I found.
Greenwell had a batting average within six points of .300 every season except for 1992 and 1994, the former in which he battled injuries, and the latter shortened by strike. He also holds the American League record for game-winning RBI in a season, with 23 in 1988. (Though GWRBI was discontinued as an official stat the next year, I find it quite interesting. I'll try to do a FanPost on it at some point). That season, he was also a runner-up to José Canseco for MVP. '88 was definitely his best season, hitting to a .325/.416/.531 line, with 192 hits and 119 RBI. In a franchise with a long legacy of outstanding left fielders, I think that Mike Greenwell could certainly fit into that group.