In fact, NHL teams sometimes play with just five defensemen, or even as many as seven. This is normal when a team cannot call up a player from the AHL—specifically, a player on a "one-way" contract who cannot be sent back down without clearing waivers—or when a player is sidelined by only a short-term injury. What you’re saying is absolutely true; the AI doesn't necessarily keep backup players for every specific position on the roster. Instead, the "scratch" group might consist of three forwards, meaning that if a defenseman gets injured, a forward has to fill in (whereas in real life, the team would shift to using two defensive pairings, with one player rotating in on the fly).