The Stormers ran in six tries to claim a bonus-point victory and end the Sharks' perfect start to the campaign. The visitors crossed three times themselves but left empty-handed after a first half that swung decisively against them.
Pietersen revealed injury concerns to flyhalf Vusi Moyo and Springbok lock Eben Etzebeth, who both left the field while the score was tied. Their absences, combined with two yellow cards, shifted the momentum firmly towards the hosts.
"Vusi has a dislocated shoulder, and he tried to fight it off," he told reporters.
"Eben, it looks like a knee or upper leg. I'm not a doctor, but he couldn't run, so we needed to make a replacement.
"When we lost Vusi and Eben, the score was still 8-8 and in the balance.
"We had a scrum penalty against us on our ball. We got two yellow cards also in the first half, so it put us under massive pressure. Losing momentum with two key guys.
"A No.10, obviously, and Eben with the forwards. While defending with 13 men is always going to be a tough ask."
Flankers Matt Romao and Manu Tshituka were both sin-binned before half-time, with Romao's yellow card coming for a dangerous tackle.
"It's quite a technical penalty for a yellow card. Romao tried to wrap his arms around him, but it looks dangerous. Obviously, when he came off, we didn't react quickly enough," Pietersen explained.
Despite the setbacks, the coach found reasons for encouragement in his team's response.
"There are some positive things. However, as a coach, you always see the negative things. It is what it is," he said.
"I think we need to look deeper into what we can do better. Nonetheless, there is some good stuff we can look at.
"The team have really shown resilience. A few years ago, we would easily lose by 60 points with 13 men, but we showed character and should build on that."
Pietersen also paid full credit to a well-drilled Stormers outfit.
"I think we shouldn't forget what Stormers team we played against. They're a team that won the URC. They played two finals," he said,
"Last year, they made a semi-final. We shouldn't forget what the Stormers have. They're cool players. They made very few mistakes. We made a lot of mistakes. We have to give credit to the Stormers."
The Sharks will begin their overseas campaign with an opportunity to test their squad depth, with JP Pietersen naming a 28-man group for three successive matches in France, Wales and Ireland.
Bulls fly-half Luan Giliomee revealed that joining the Bulls has been a positive and welcoming experience, praising the support he has received from teammates and the coaching staff.
Head coach Ivan van Rooyen and captain Francke Horn insist the Lions will not let their 57-15 defeat to the Bulls in the United Rugby Championship set the tone for the rest of the season.
Two one-sided South African derbies headlined the third round of the United Rugby Championship, with Bulls and Stormers players dominating the team of the week.
