According to this tweet from Carrie Muskat, the Cubs have claimed right-handed reliever Sandy Rosario from the Red Sox. The Red Sox had claimed Rosario from the Marlins on Monday, so this is Rosario's third team in three days.
Rosario is a hard-throwing reliever with command problems. His slider is serviceable and he doesn't have a third pitch. In three brief stints with the Marlins in 2010 through 2012, Rosario pitched in ten games for a total of 7⅔ innings. His major league career ERA is 15.26. But he has had some success in the minor leagues. Last season for the Marlins Triple-A team in New Orleans, he pitched 26 innings and struck out 24 batters and walked only two. He was the Zephyrs closer and had 13 saves. His ERA in New Orleans was 1.04.
According to cubs.com, this now brings the Cubs 40-man roster to 40 players. Ian Stewart is not yet listed, so unless they've got some trick or there's something about Stewart's contract that we don't know, then a corresponding move will have to be made when Stewart is added.