i believe this is the answer to your question. \n\n3.08\n(a) If no announcement of a substitution is made, the substitute shall be considered as having entered the game when_ (1) If a pitcher, he takes his place on the pitcher's plate; (2) If a batter, he takes his place in the batter's box; \n(3) If a fielder, he reaches the position usually occupied by the fielder he has replaced, and play commences; (4) If a runner, he takes the place of the runner he has replaced. (b) Any play made by, or on, any of the above mentioned unannounced substitutes shall be legal.