Puk was diagnosed with a mild left shoulder strain Tuesday, Martin Gallegos of MLB.com reports.
The 24-year-old will be shut down from throwing until being reevaluated Thursday or Friday. Puk already underwent an MRI which revealed no structural issues, but it's nonetheless a discouraging spring-training development for the young left-hander. Puk's availability for Opening Day will be in limbo until he receives an official timetable for his return to the mound.