The result in the Jukskei derby ended the Lions' perfect start to the campaign and brought them crashing back down to earth after home wins over Leinster and the Ospreys.
They were blown away by a Bulls side packed with returning Springboks, who led 31-10 at half-time and never looked back.
Van Rooyen was quick to credit the opposition while turning the spotlight firmly on his own team's shortcomings.
"I don't want to take anything away from the Bulls. I think they were exceptional. It felt like we played against 10 or 12 Springboks today and they were good," he said.
"But in saying that, with all the respect in the world, I thought we were really below par. A lot of the fixes will be on us."
A pivotal moment arrived just after the 25-minute mark when Springbok scrumhalf Morne van den Berg was shown a yellow card for a cynical breakdown infringement.
"The yellow card was quite costly. We let in two soft first-phase tries in that period," Van Rooyen said.
"We couldn't hold onto the ball or build pressure in attack. We have still got to get out the blocks."
The coach highlighted how quickly the scoreboard got away from his side, with soft, low-phase scores leaving the Lions chasing the game from the outset.
"If you give the Bulls 31 points in the first half with three tries that didn't go past two phases, that puts you under pressure," he said.
"We need to be more focused, more clinical, and if things go a bit bad, just calm down and get cool heads."
Despite the scale of the loss, Van Rooyen insisted one result would not define the season.
The Lions have worked hard to establish a platform early in the campaign and the coach made clear they know the standards required to stay competitive.
"It will be a shame if this one result determines the whole of the next seven months," he added.
"We worked hard to get where we are and we know what we need to do."