The injury has failed to settle, and, after further assessment, it has been established that the 21-year-old is likely to be unavailable for selection for around six weeks.
Gregor said: “It’s clearly frustrating for both Duncan and ourselves that he’s missing out on some big games.
“However, with many more important fixtures coming up over the course of the season, it’s very important that he gets himself right, and I know he’ll be working hard with our medical team to return to the fray in due course.”