Sharks eye Lions' scalp
It was the first away win this season for the Pretoria-based Bulls and the first home loss for the Sharks, who have already secured a quarterfinals place.
Du Preez dismissed the notion that the four-week June break and the absence of some Springboks were contributing factors to his team's loss, pointing to the fact that the Bulls face the same 'obstacles'.
He spoke of a "lack of intensity" and said it was "really disappointing".
"We looked really disjointed," Du Preez said, adding that injury disruptions had a role to play.
However, he felt a bigger problem was the "attitude" – or lack thereof – of the players.
The Sharks now have a bye and then face the high-flying Lions – who at the weekend secured a top-two finish – in Durban on July 15.
"We have to go back to the drawing board," Du Preez said, adding that the disappointing performance came at the end of a good week of training.
"To come out here and produce a performance like that, is just unacceptable."
Statistics from the match will be of big concern Sharks to Du Preez, as his team faces a possible last-eight clash with the unbeaten Crusaders in New Zealand.
There were 17 handling errors and 21 missed tackles against the Bulls.
Sharks skipper and loose forward Philip van der Walt said: "We committed too many handling errors and could not retain possession long enough to threaten the Bulls.
"The team battled to get over the advantage line and perhaps there was an element of rustiness after the month-long break for the Test series.
"We need to take a long, hard look at ourselves and come back with a bang when we host Golden Lions in two weeks."
@rugby365com
