With Tongan Siua Halanukonuka out for another six weeks with a foot injury, Glasgow are lacking depth at tighthead with the Heineken Champions Cup starting next month.
"We think he'll be out for five to six months," Rennie said of Fagerson. "We are hoping maybe five or four and a half. It was a clean break and the operation went well but we are not going to push him. It's a big season with a World Cup following."
Fagerson, who was already out of the autumn Tests and the start of Glasgow's Pool 3 campaign, is likely to miss most if not all of the Six Nations again, following his disappointment earlier this year when a freak gym injury as he dropped a weights bench on his foot limited him to just one bench appearance in Rome at the end of the campaign.
"Zander has been superb," said Rennie. "The scrum has gone well but the quality of his carrying and cleaning and tackling has been great. There is a bit of mongrel about him and we'll miss that."
With Halanukonuka also sidelined, Rennie is left with youngsters D'arcy Rae and Adam Nicol as his tighthead options, and admitted that cover was being sought to bolster the squad ahead of Europe. "Siua has a foot injury. D'arcy started last week and we have young Adam Nicol but we are a little thin and at a stage where we might have to look at bringing someone in," continued the coach.