According to MLB rules,
Upon being optioned to the Minor Leagues, a position player must remain there for a minimum of 10 days before he is eligible to be recalled to the Major League roster. For pitchers, the minimum is 15 days
I wonder why this rule has never been implemented in OOTP, especially when PT mode has something similar when sending down players. Without it, one can keep an eligible pitcher down between appearances. I'll definitely adhere to this rule but that requires keeping track of the transactions.