A pinch hitter is up to bat. A runner on first gets thrown out at second for the third out of the inning. The pinch hitter is not substituted for the player he was pinch hitting for. In the next inning who is up to bat the pinch hitter who then completes his bat he did not finish the last inning or the player he was pinch hitting for who had taken the field the inning before?
According to rule 5.10(d), "A player once removed from the game may not re-enter that game." The original player is removed from the game as soon as the pinch hitter enters the game. Thus, the original player may not take the field.
If there was a pinch hitter on the field, the player he replaced is no longer eligible to go back because he was already substituted and the one who pinched hit for him will replace him on the batting order.