Anybody know the rule?

In last night's game, Anderson came back after sitting out only 1 play instead of 3. I'm not complaining that the refs missed it but, does anyone know what the penalty should have been?

Illegal substitution, 10 yard penalty ?

as per Section 11 - Article 3, it is a 10 yard penalty with the down replayed if the official doesn't catch them before the play starts.

Was he attended to on the field by the medical staff? I never caught the game.

Yes.
The trainers came on to the field which means he must sit out 3 plays. He was back on the field after 1 play and there should have been a penalty. The refs missed it.

On a side note.
I always felt that an offensive player should sit out 3 offensive plays....a defensive player, 3 defensive plays.

The officials blew that; they never should have let him back onto the field in the first place. It should have been a 10-yard penalty, though with the incomplete pass on the play the Als may have declined the penalty if it had been called.

But they were correct in not allowing a challenge. Penalties are not reviewable. Frustrating, but the Cats have suffered as much from that restriction as anybody.

As another side note:
It is possible for a player who has gotten medical attention to return in less than three plays. If the player is injured as a result of an infraction for which a penalty flag has been thrown, the player is not required to leave the field at all. Though usually they come off the field for the simple fact that, well, they're INJURED!

Thats true but, it is at the refs discretion

...i think if a call like that is missed....the player should have to sit out 4 plays ...that being added to the ones he already missed (if and when the refs. or opposition catch it).....there's no excuse for a club not to know or follow the rules....