Found this under Wikipedia. Not seeing any exemptions for full season injuries. Maybe someone else will have a better idea than I.
Players are eligible for selection in the Rule 5 draft if they are not on their major league organization's 40-man roster and:
were 18 or younger on the June 5 preceding their signing and this is the fifth Rule 5 draft upcoming; or
were 19 or older on the June 5 preceding their signing and this is the fourth Rule 5 draft upcoming.
These exemption periods (one year longer than those in effect previously) went into effect as part of a new Collective Bargaining Agreement in October 2006. The change took effect immediately, exempting many players from the 2006 Rule 5 draft even though they had been signed in some cases more than four years before the new agreement came into effect. Prior to the rule change, players were exempt from the first two or three Rule 5 drafts held after their signing (regardless of the year they were drafted), rather than from the first three or four Rule 5 drafts after their signing.