Luckily, several of those players have been able to step in immediately and produce at the major league level.
Daniel Nava has been a revelation since being pressed into action May 10, hitting .294 with a .411 on-base percentage and .462 slugging percentage. Fellow outfielder Ryan Kalish has not played as much as Nava — he's appeared in just 13 games so far — but many believe that the 24-year-old lefty could have the highest ceiling of the Sox' youngsters. Kalish has struggled to stay healthy over the past two seasons, but when he has been on the field, he has produced, hitting .345 with five home runs in 15 games with Pawtucket this season before his call-up to the big club.
Pitcher Aaron Cook began this season in the minors, but after a 3-0 record and 2.41 ERA in six starts in Pawtucket, he was promoted to Boston as a spot starter when Clay Buccholz went on the DL. Cook has looked good in two starts since then, winning both, the latter a two-hit shutout of the Mariners on Friday. Few expected this kind of success from Cook, who went 3-10 with a 6.03 ERA last season in Colorado.
And then, of course, there's third baseman Will Middlebrooks. The 6-foot-4 Texan's rapid ascent made Kevin Youkilis expendable, and Middlebrooks has put together an impressive line of .298 BA, .335 OBP and .538 slugging to go with 10 home runs and 37 RBIs in 48 games.
The ability of Sox call-ups to come into the lineup and play well has no doubt been a big factor in the team's 30-18 record since May 11. But which player has had the most surprising season?