It is all about statistics if you ask me. I am in the 13th season. So far I had seasons with 0 injuries and season with injury storms that went up to 14 injuries in the last half. I am using the same strategy for almost all 13 seasons so it is all random if you ask me. If an injury storm hits you, better prepare. It will hit hard. 14 injuries in about 10 days is my max (used over 150 red packs to keep the team alive).