The Lions were down 32-17 at half-time, but rallied in the second half with tries from Edwill van der Merwe and JC Pretorius.
Van Rooyen says the belief in the squad is still strong heading into their final three pool matches.
"I still believe – we've got three games, so three wins that we need to get here. The reality is that all the teams are playing each other and we'll sort each other out," said van Rooyen.
"So we knew we had four games against teams that are in competition for that six set of eight spots, and we're still alive – I believe it, we'll speak it, but we need to be better."
The Lions picked up a bonus point for scoring four tries, but van Rooyen admits too many handling errors and missed opportunities, even with Benetton down to 14 men three times in the second half, proved costly.
"The penalties, I think something like eight times of [giving away] a penalty within three phases when we were defending, and then they get into three-plus positions, advancing up the field and dictating the tempo of the game," he added.
"There were too many errors from our side, and again, good fight, again, trying to fight back, but I mean if we're that far behind then time catches up with you."