Can a player react to Watchful Sentry's deathknell trigger by playing a hidden Teemo that was face-down on the battlefield, and how does this resolve?

Ruling: You can technically play Teemo in reaction to the deathknell trigger, but Teemo becomes essentially a vanilla unit in this situation because he can only be played after players and units have had attacker and defender designations removed.

Nuances:

• This is a weird edge case situation that may have rules clarifications in the future
• The interaction becomes even more complex with Teemo Strategist
• Players should check with their head judge when this situation arises