The Parisians lost 31-17 on Sunday condemning back-rower Kolisi to a trophyless season in his first campaign with the club after April's Champions Cup last-16 loss to eventual winners Toulouse.
"He put on a few kilos, lost his form and yesterday he was invisible," Lorenzetti told specialist website Rugbyrama.
"We missed him," the 76-year-old added.
Kolisi, who told AFP last week he had gained 5kg (11 lbs) before losing it again, arrived in the French capital after lifting a second straight Rugby World Cup with the Springboks.
The 32-year-old was brought in by six-time French champions Racing on a two-year deal as a replacement for, among others, ex-France flanker Wenceslas Lauret.
"At back-row we had been used to seeing an ultra-present warrior like Wenceslas Lauret," Lorenzetti said.
"For now, Siya isn't making us forget him.
"But we think next year, it will change," the Frenchman added.
Racing, without a trophy since 2016's Top 14 success, have signed ex-England captain Owen Farrell as well as France forwards Demba Bamba and Romain Taofifenua for next season.
They have reportedly agreed a deal for Kolisi's fellow South African, Stormers' back-rower Hacjivah Dayiamani.
