I've seen the words "back" and "middle-body" mentioned towards his status.
And I know he came into camp with that injury.
Also - to read between the lines of Torts comments, he seems annoyed to be asked.
All that together, it was (likely) an off-season (non-hockey) injury (Boone "declined to be interviewed about his status")... and possibly the organization is frustrated.
I know that Boone is known as a hard-working farm boy - and I sort of wonder if he wrenched his back doing some strenuous activity.
Both my dad and I have had serious back issues and the time table for a messed up back is completely unknown... but a serious back-issue (you KNOW if you have one)... is a minimum of two months.
The muscles get all ****ed up and it takes them a long time to loosen back to normal.
That's my speculation - and I've haven't read anything else even remotely hinting as to why/when/how it happened.