Boone revealed that Severino has a loose body in his pitching elbow. He said that it's too early to determine whether the right-handed pitcher will be ready for the beginning of the regular season.
"We'll see," Boone told reporters. "First things first is trying to get our arms around exactly what is going on in there, and then hopefully we'll be in a position to get him going again."
Severino's forearm issue dates back to the AL Championship Series in October. Boone said the pitcher was cleared and would have started a potential Game 7 of the ALCS.
The Yankees had Severino undergo two MRIs and a CT scan in New York after complaints of soreness resurfaced this off-season. Severino believed he was close to full health, but a new recurrence of that forearm discomfort prompted the club to shut him down.
"There's an unknown right now," Yankees general manager Brian Cashman said Thursday. "It could be small and a timing issue or it could be more significant than that, because we haven't determined what we're dealing with yet."
Severino became the second pitcher to be shut down by the Yankees this spring after reporting the ailment in his throwing arm. New York will be without starting pitcher James Paxton through at least April after he underwent surgery to remove a cyst.
Severino, a two-time All-Star selection, posted a 1-1 record and 1.50 ERA with 17 strikeouts in three starts last season.