The Bulls fell to a disappointing 32-19 defeat to the Cape outfit in the United Rugby Championship but the bad news didn't stop there as it now appears De Klerk could be facing a significant injury as well.
The utility back has been one of the Bulls' best players this season but his game against the Stormers was cut short when he was forced off just before half-time.
"There is something wrong with Sebastian's foot; it could be a broken bone. We will get more clarity later," Ackermann said.
The injury could not come at a worse time for De Klerk. The centre has reportedly caught the eye of Springbok head coach Rassie Erasmus, who is busy preparing for a bumper international season in 2026 and a lengthy spell out injured will not do his chances of a call-up any good.
The Bulls will also be without inside centre Harold Vorster, who took a knock to the head against the Stormers and failed a concussion test towards the end of the encounter.
There is still some hope he will be ready in time for the clash against Cardiff this weekend but the Bulls are not going to take any chances with a head injury.
"Harold failed his concussion test right at the end. We will have to see if he will have enough days to turn it around, but he failed the one he did after the game, so it does not look good," Ackermann said.
Ackermann conceded the Bulls' margin for error has shrunk dramatically due to their poor form to start the season.
"We have put a lot of pressure on ourselves. Our margin for dropping points is getting less and less. If we had won this home game, it would have eased the pressure on our away games," he said.
"But now there is more pressure on us to beat Cardiff, and then we've got Munster left, followed by a month away from home on the road, for the EPCR as well as the remaining away URC games. In short, what we need now is action, not sulking."