South Africa Rugby World Cup-winning coach Jake White was recovering on Monday after successful abdominal surgery, the Bulls franchise said.
The 59-year-old had been unwell during a visit to Wales last week and was unable to attend a United Rugby Championship victory over the Dragons.
When the Bulls returned to South Africa on Sunday, White complained of severe stomach cramps and underwent surgery the same day in a Pretoria hospital.
"We are extremely confident that Jake is receiving the best medical attention and wish him a full and speedy recovery," said Bulls chief executive Edgar Rathbone.
"Special thanks to the hospital team, doctors and all those who rushed to offer their expertise and help Jake."
White will miss several matches, starting with a European Champions Cup pool clash against Exeter Chiefs in Pretoria on Saturday.
He masterminded the Springboks' 2007 World Cup campaign, which culminated in a 15-6 victory over England in the final at the Stade de France in Paris.
He went on to coach clubs and franchises in Australia, South Africa, France and Japan, and was technical advisor to Tonga, before joining the Bulls three years ago as director of rugby.