Ellis picked up an ankle injury during the warm-up at Headingley before the visitors lost the opening match of the season 36-6 against the champions.
"It started bad and got worse," Gentle told Sky Sports News. "It's tough losing your leader like that but we're not going to use that as an excuse.
"We started playing like individuals when Leeds went up a gear. It's a massive reality check for us, it shows how far we're off the top teams in this competition."
Hull return to Super League action next Friday when they welcome Bradford Bulls to the KC Stadium.