There had been some initial concern surrounding Castro because it was also mentioned that he left with forearm tightness. That usually means bad things for the elbow, but that's not something the Blue Jays are currently concerned about.
"The forearm was related to the finger," Blue Jays general manager Alex Anthopoulos explained in an email. "Index finger gets tight gripping the baseball every pitch and you start feeling a little tight in your forearm."
Anthopoulos added that Castro doesn't require an MRI on his right elbow and the focus will instead be on the index finger. There is no immediate timetable for his return, and the Blue Jays won't know more until he undergoes further evaluations.
Castro began the year in Toronto's bullpen and took over the full-time closer duties during the first week of the season. He struggled by allowing seven runs over 12 1/3 innings before getting sent to the Minors.
The 20-year-old had been using his time in the Minor Leagues to get stretched out as a starter. He tossed a season-high five innings without an earned run on May 23 against Toledo. But Castro's next outing lasted just four outs before he had to come out of the game.
The Dominican native has a 3.09 ERA in five appearances with the Bisons this season.
Gregor Chisholm is a reporter for MLB.com. Read his blog, North of the Border, follow him on Twitter @gregorMLB and Facebook, and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.