Perhaps the terminology of day-to-day is misleading. Read this section of the manual:

"Day to Day Injuries: Injuries which are caused by overuse or tiredness. The length of injuries is affected by the severity of the tiredness or overuse. These injuries are enforced and present in Launch leagues as well as perpetual leagues.
This type of injury occurs when a player is used in a game while overused or tired.
This can occur if player appears in a game with one of the following status'; Overused, Very Overused, Tired, or Very Tired.
Additionally, a Day to Day injury occur if a Pitcher becomes tired during the course of the game. This occurs when he exceeds his BFA and the text in the game identifies him as tired.
Injuries of this type (Day to Day) are generally of a short term, but their duration can be longer if the player is extremely overused or tired. Day to Day injuries can happen to ANY player regardless of whether or not they were hurt in real life."

The manual seems to contradict itself but perhaps even a 0 injury guy can get longer term injuries when overused???

I like the quote attibuted to Patton:
"Never give directions that can be understood. Give directions that can NOT be misunderstood."

The injury explanation can definitely be misunderstood and should be reworded...whatever it is supposed to mean.