Every now & then I see someone suggest the Cubs should just put Tyler Chatwood on the DL so that he doesn't take-up a spot on the 25-man roster from a pitcher who would be more-useful to Maddon and the Cubs. The problem is,
a club cannot place a player on an MLB DL unless the club physician signs off on it - AND - (where there is no physical evidence to corroborate the diagnosis) the player consents (or"plays along," so to speak).
So an MLB DL assignment requires either some tangible confirmation evidence of injury (like an x-ray or MRI) or illness (blood test) to justify the DL assignment, or the player has to claim an injury (like "my shoulder is stiff," or "my calf hurts," or "my back is tight").