DH Plays Defense - Fed Rules

[Dave Reed]

What's the rule that says #22 is the player to be restricted? When he went to play the field, the DH was terminated, and either he or #8 is now illegally in the game. I don't see how we determine which one is illegal.


Here's a related situation:
"3.1.4 SITUATION A: DH Jones, who has been batting for F3 in the fourth position in the batting order, hits a triple in the fifth inning and sprains his ankle sliding into third base. His coach has S1 enter the game to be a pinch runner for DH Jones. How does that affect the playing status of DH Jones and F3? RULING:
When a pinch runner or pinch hitter replaces the DH, that player becomes the DH. F3 would not be affected by the substitution. However, if the DH were to play defense, F3 would have to leave the game."

This casebook play seems to suggest that #8 should have (or could have) left the game, and then #22 would just be an unannounced defensive change.

What rule/ruling says #22 rather than #8 is illegal? (I'm not arguing that #8 is a better choice; just that he is as good a choice.)