Download the D2 Manual, and read 14.2.5, which covers hardship waivers--the correct terminology for "medical redshirt". Basically, to get a hardship waiver, a player needs to suffer an injury that incapacitates him for the remainder of the season, and the injury needs to be documented by a physician (not a chiropractor or trainer) at the time of the injury. The documentation needs to clearly show that the player was unable to play, not just impaired.
The normal reason to apply for a waiver is an injury occurring in the early part of the season, after the player has competed in some games. The waiver actually adds a season of competition while still counting the partial season just played. In your first year, the injury occurred before the season began, and you didn't compete. So you didn't need a waiver because you didn't use a season of competition. Unless the injury is a torn ligament, or really nasty muscle tear, a fall injury may be considered to not be season ending, so it's possible that you weren't eligible for a waiver anyway.
Now you have a second injury, and if both injuries were documented to be season ending, then you may be able to get a waiver which would allow a 6th season, as described in 14.2.7. Your college has to request it; you can't do this on your own. I suspect that your coach believes that you don't have the necessary documentation for both seasons in order to make a successful waiver request.
