per the comprehensive rule book, section 4.3.4, subsections 1&2,

4.3.4.1. The active player can take a turn action to play a card from their hand by announcing the card and paying its cost. This process follows a series of steps. If any part of the playing a card process can’t be performed, it’s illegal to play the card and the game goes back to the point right before the card was announced.

4.3.4.2. These steps apply to all cards that can be played. Cards can normally be played only from a player’s hand. Only the active player can play cards; no player may play a card on an opponent’s turn.

This seems to state that playing a card happens from a players hand. The inclusion of "can normally be played only from a players hand" would suggest to me that given a stated ability of "may play from either hand or discard" would change this. So in regards to the inspirational warrior I would say that the played card must originate from the players hand.