Gil told Boone during a bullpen session on Friday that he felt a constant stiffness in his throwing shoulder that he couldn't loosen.
"It's at least a six weeks no-throw, we know that for sure, but he's still got to be examined by doc and get the other opinions on everything there," said Boone. "He's been down this road before. But I think he's at least right now in the right frame of mind."
"It sucks, man. I don't even know what to say to put it into words," said teammate and starting pitcher Marcus Stroman on Gil's injury. "He was a huge part of this team last year. Incredible, incredible season, and we're going to need him. We're going to need him at some point in order to go where we want."
Gil won the American League (AL) Rookie of the Year last season after posting a 15-7 record with a 3.50 ERA along with 171 strikeouts and an AL-leading 77 walks in 151.2 innings pitched.
The Dominican Republic could potentially miss anywhere from four to six starts if he does return around mid-April.
After Tommy John surgery, Gil missed most of the 2022 and 2023 seasons before rebounding this past year.