Alex Wilson, RP
Alex Wilson was Boston's #11 prospect according to Baseball America, and #15 at Baseball Prospectus, heading into 2012. He's since made a shift to relief, where his slider and fastball -- both deemed tops in Boston's farm by Baseball America -- have been very effective for him, as expected. He's thrown 55 innings in relief, striking out 56 batters against 24 walks, posting a 3.10 ERA in his first experience in the role. He could stand to walk fewer batters, but this is still a very solid first go of things.
Wilson needs to be added to the 40-man roster by mid-November, lest he be lost to the Rule 5 draft. And, given the season he has had, and his proximity to the majors, he absolutely would be claimed. Whether Boston adds him in time for a September call-up as a reward (and an audition) in the majors is unknown right now, but he'll be on their 40-man by the time he needs to be regardless.
Alex Hassan, OF
Hassan is also in danger of being selected in the Rule 5 draft, but, unlike Wilson, it's not likely he'll be put on the 40-man roster to protect from this. Hassan has some skills, but he's a very incomplete player, even when things are going right. He's a problem defensively, and while his patience lands him lofty on-base percentages in the minors, his lack of ability to hurt the opposing pitcher will result in more outs against major-league pitching.
Because of this, though, someone might not be willing to put a Rule 5 claim in on Hassan, as they would need to keep him on their major-league roster during the 2013 season, or offer him back to Boston. Maybe someone like the Astros, who don't exactly have much to lose, would take a flyer on him, but otherwise, it's difficult to see him anywhere but Pawtucket again next season.
Dan Butler, C
Butler isn't a prospect, but he's a defensive-oriented catcher, and the highest in the Red Sox organization that isn't currently in the majors. He's struggled in his brief time in Pawtucket, but he's not there to hit. He won't end up on this year's 40-man roster unless Boston desperately wants a third catcher in town in September, but as of now, he's the most likely to get that designation. Christian Vazquez might need to be protected from the Rule 5 draft, but, like with Hassan and Wilson, Boston has a few months before needing to do that, at which time a few of their current players will be free agents, and no longer taking up roster space.
Butler actually hit a bit at Double-A -- not anything special, but enough for a sure-handed backstop. If he could get to that level, he'd fill the kind of role that someone like Luis Exposito used to, before Boston designated him for assignment earlier this year.