Bulls defence coach Pine Pienaar has taken responsibility for some of the problems that contributed to the team losing three of their opening four Super rugby matches.
Already up against it, they face the Chiefs in Hamilton at the weekend.
In their last match they lost 38-14 to the Blues, missing 24 tackles and Pienaar said they must improve.
"I am not happy with the defence because to concede six tries in a match is unacceptable to me.
"I take full responsibility because it is my area and I am not happy with it.
"Players are always going to be under pressure if they leave space between their inside and outside defender, but that is something we will work on," he said.
"We want to work on the mistakes we created."
The Bulls have confirmed that prop John-Roy Jenkinson, who suffered a hamstring injury in the defeat of the Blues in Albany on Saturday, will return home for further treatment and will be replaced by Conraad van Vuuren.
No8 Hanro Liebenberg will have a late fitness test after a pectoral muscle injury.
The Lions have taken the unusual step of naming a provisional squad to play the Sharks at Ellis Park on Saturday.
Utility back Lionel Mapoe and prop Johannes Jonker are hoping to be fit for the game but Jaco van der Walt, Ruan Combrinck, Howard Mnisi and Dylan Smith weren't considered because of injury.
Sylvian Mahuza will attend his father's funeral this week.
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